Tuesday, 8 March 2016

NHRP flags

NHRP: Examples


The following is sample output from the show ip nhrp command:

Router# show ip nhrp 



10.0.0.2 255.255.255.255, tunnel 100 created 0:00:43 expire 1:59:16


 Type: dynamic Flags: authoritative 


 NBMA address: 10.1111.1111.1111.1111.1111.1111.1111.1111.1111.11 


10.0.0.1 255.255.255.255, Tunnel0 created 0:10:03 expire 1:49:56


 Type: static Flags: authoritative 


 NBMA address: 10.1.1.2 

The fields in the sample display are as follows:

The IP address and its network mask in the IP-to-NBMA address cache. The mask is always 255.255.255.255 because Cisco does not support aggregation of NBMA information through NHRP.

The interface type and number and how long ago it was created (hours:minutes:seconds).

The time in which the positive and negative authoritative NBMA address will expire (hours:minutes:seconds). This value is based on the ip nhrp holdtime command.

Type of interface:

dynamic—NBMA address was obtained from the NHRP Request packet.

static—NBMA address was statically configured.

Flags:

authoritative—Indicates that the NHRP information was obtained from the Next Hop Server or router that maintains the NBMA-to-IP address mapping for a particular destination.

implicit—Indicates that the information was learned not from an NHRP request generated from the local router, but from an NHRP packet being forwarded or from an NHRP request being received by the local router.

negative—For negative caching; indicates that the requested NBMA mapping could not be obtained. When NHRP sends an NHRP resolution request it inserts an incomplete (negative) NHRP mapping entry for the address in the resolution request. This is to keep the router from triggering more NHRP resolution requests while this NHRP resolution request is being resolved and the IKE or IPsec tunnel created.

unique—NHRP registration request packet had the "unique" flag set (on by default). This means that this NHRP mapping entry cannot be overwritten with a mapping entry that has the same IP address but a different NBMA address. When a spoke has a statically configured outside IP (NBMA) address this flag is used to keep another spoke that is misconfigured with the same tunnel IP address from overwriting this entry. If a spoke has a dynamic outside IP (NBMA) address then you configure ip nhrp registration no-unique on the spoke to clear this flag. This flag then allows the registered NHRP mapping entry for that spoke on the hub to be overwritten with a new NBMA address. This is necessary in this case since the spoke's outside IP (NBMA) address may change at any time. If the "unique" flag was set, then the spoke would have to wait for the mapping entry on the hub to time out before it could register its new (NBMA) mapping.

registered—The mapping entry was created from receiving an NHRP registration request. Registered mapping entries are dynamic entries, but they will not be refreshed through the "used" mechanism. These entries are refreshed by receiving another NHRP registration requests with the same tunnel IP to NBMA IP address mapping. The NHC must periodically send NHRP registration requests to keep these mappings from expiring.

used—When data packets are process-switched and this mapping entry was used, the mapping entry is marked as used. The mapping data base is checked every 60 seconds. If the used flag is set and there are more than 120 seconds left in the expire time, the used flag is cleared. If there are fewer than 120 seconds left in the expire time, then this mapping entry is "refreshed" by sending another NHRP resolution request.





router—NHRP mapping entries that are for a remote router itself for access to a network or host behind the remote router are marked with the router flag.
local—NHRP mapping entries that are for a network's local to this router (serviced by this router) are marked with the local flag. These entries are created when this router answers an NHRP resolution request with this information and are used by the rouer to store the tunnel IP address of all of the other NHRP nodes to which this router has sent this information. If for some reason this router loses access to this local network (it can no longer service this network) it will send an NHRP purge message to all remote NHRP nodes listed in the 'local' entry (this list is not visible) to tell the remote nodes to clear this information out of their NHRP mapping tables. This 'local' mapping entry times out of the local
NHRP mapping database at the same time that this information (from the NHRP resolution reply) would time out of the NHRP mapping database on the remote NHRP nodes.
implicit—NHRP mapping entries that were learned by the local node by using the source NHRP mapping information from an NHRP resolution request or reply.
(no socket)—NHRP mapping entries for which the router does not need nor want to trigger IPsec to set up encryption, because the router does not have data traffic that needs to use this tunnel. If later on there is data traffic that needs to use this tunnel it will be converted from a "no socket" to a "socket" entry and IPsec will be triggered to set up the encryption for this tunnel. Local and implicit NHRP mapping entries are always initially marked as "no socket."
NHRP by default caches source information from NHRP resolution request or replies as they go through the system. In order to allow this caching to continue, but not have the entry create an IPsec socket they are marked as (no socket). If this was not done there woudl be extra IPsec sockets from the hubs to the various spokes that either were not used are were used for only one or two packets while the spoke-to-spoke tunnel was being built. Data packets and NHRP packets that arrive on the tunnel interface and are forwarded back out the tunnel interface are not allowed to use the (no socket) NHRP mappings for forwarding. Because in this case, the router is an intermediate node in the path between the two endpoints and we only want to create short-cut tunnels between the entrance and exit point of the DMVPN (NBMA) network and not between any intermediate nodes. If at some point the router receives a data packet that has a source interface that is not the tunnel interface and it would use the (no socket) mapping entry, the router converts the (no socket) entry to a (socket) entry. And in this case, this router is the entrance (or exit) point of the NBMA (for this traffic stream).
Also these (no socket) mapping entries are marked (non-authoritative); only mappings from NHRP registrations are marked (authoritative). The NHRP resolution requests are also marked (authoritative), which means that the NHRP resolution request can be answered only from an (authoritative) NHRP mapping entry. A (no socket) mapping entry will not be used to answer an NHRP resolution request and the NHRP resolution request will be forwarded to this nodes NHS.
nat—This setting is on NHRP mapping entries that are from NHRP registration packets. This indicates that the remote node (NHS client) supports the NHRP NAT extension type for supporting dynamic spoke-to-spoke tunnels to or from spokes behind a NAT router. This flag does not mean that the spoke (NHS client) is behind a NAT router.
NBMA address—Nonbroadcast multiaccess address. The address format is appropriate for the type of network being used (for example, GRE, Ethernet, SMDS, or multipoint

DMVPN Dynamic Tunnels Between Spokes Behind a NAT Device

DMVPN Dynamic Tunnels Between Spokes Behind a NAT Device


The DMVPN: Dynamic Tunnels Between Spokes Behind a NAT Device feature allows Next Hop Resolution Protocol (NHRP) spoke-to-spoke tunnels to be built in Dynamic Multipoint Virtual Private Networks (DMVPNs), even if one or more spokes is behind a Network Address Translation (NAT) device.


Restrictions for DMVPN Dynamic Tunnels Between Spokes Behind a NAT Device

In order for spokes to build tunnels between them, they need to know the post-NAT address of the other spoke.
Consider the following restrictions when using spoke-to-spoke tunneling in NAT environments:
  • Multiple NAT translations --A packet can go across multiple NAT devices in a nonbroadcast multiaccess (NBMA) DMVPN cloud and make several (unimportant) translations before it reaches its destination. The last translation is the important translation because it is used to create the NAT translation for all devices that reach a spoke through the last NAT device.
  • Hub or spoke can be reached through pre-NAT addresses --It is possible for two or more spokes to be behind the same NAT device, which can be reached through a pre-NAT IP address. Only the post-NAT IP address is relied on even if it means that a tunnel may take a less desirable path. If both spokes use NAT through the same device, then a packet may not travel inside-out or outside-in as expected by the NAT device and translations may not occur correctly.
  • Interoperability between NAT and non-NAT capable devices --In networks that are deployed with DMVPN, it is important that a device with NHRP NAT functionality operate together with non-NAT supported devices. A capability bit in the NHRP packet header indicates to any receiver whether a sending device understands a NAT extension.
  • Same NAT translation --A spoke’s post-NAT IP address must be the same when the spoke is communicating with its hubs and when it is communicating with other spokes. For example, a spoke must have the same post-NAT IP address no matter where it is sending tunnel packets within the DMVPN network.
  • If one spoke is behind one NAT device and another different spoke is behind another NAT device, and Peer Address Translation (PAT) is the type of NAT used on both NAT devices, then a session initiated between the two spokes cannot be established.
One example of a PAT configuration on a NAT interface is:
ip nat inside source list nat_acl interface FastEthernet0/1 overload

Information About DMVPN Dynamic Tunnels Between Spokes Behind a NAT Device

The following sections describe how DMVPN: Dynamic Tunnels Between Spokes Behind a NAT Device allows spoke-to-spoke tunnels to be built even if one or both spoke devices are behind a NAT device:

DMVPN Spoke-to-spoke Tunneling Limited to Spokes not Behind a NAT Device

NAT allows a single device, such as a router, to act as agent between the Internet (or “public network”) and a local (or “private”) network, and is often used because of the scarcity of available IP addresses. A single unique IP address is required to represent an entire group of devices to anything outside the NAT devi ce. NAT is also deployed for security and administration purposes.
In DMVPN networks, spoke-to-spoke tunneling is limited to spokes that are not behind the NAT device. If one or both spokes are behind a NAT device, a spoke-to-spoke tunnel cannot be built to or from the NAT device because it is possible for the spoke-to-spoke tunnel traffic to fail or be lost “black-holed” for an extended period of time.
The diagram below and the following sections describe how DMVPN works when spoke-to-spoke tunneling is limited to spokes that are not behind a NAT device.
Figure 1. Implementation of DMVPN Spoke-to-spoke Tunneling Limited to Spokes Not Behind a NAT Device

NHRP Registration

When an NHRP registration is received, the hub checks the source IP address on the encapsulating GRE/IP header of the NHRP packet with the source NBMA IP address, which is contained in the NHRP registration packet. If these IP addresses are different, then NHRP knows that NAT is changing the outer IP header source address. The hub preserves both the pre- and post-NAT address of the registered spoke.

Note


If encryption is used, then IPsec transport mode must be used to enable NHRP.

The following show ip nhrp command output example shows the source IP address of the NHRP packet and tunnel information for Spoke B in the figure above:

Note


The NBMA (post-NAT) address for Spoke B is 172.18.2.1 (the claimed NBMA (pre-NAT) source address is 172.16.2.1).

Router# show ip nhrp
10.0.0.11/32 via 10.0.0.11, Tunnel0 created 00:00:21, expire 00:05:38
  Type: dynamic, Flags: authoritative unique registered used
NBMA address: 172.18.2.1
   (Claimed NBMA address: 172.16.2.1)

NHRP Resolution

The following describes the NHRP resolution process between Spoke A and Spoke B shown in the figure above, where Spoke B is behind a NAT device with pre-NAT address of 172.16.2.1 and a post-NAT address of 172.18.2.1:
  • The NHRP table entry for Spoke B on the hub contains both the post-NAT and pre-NAT addresses. When the hub receives an NHRP resolution request for the VPN address (tunnel address) of Spoke B, it answers with its own NBMA address instead of Spoke B’s NBMA address.
  • When the hub receives an NHRP resolution request sourced from Spoke B for any other spoke, the hub also answers with its own NBMA address. This ensures that any attempt to build a spoke-to-spoke tunnel with Spoke B results in the data packets being sent through the hub rather than through a spoke-to-spoke tunnel.
For example:
    • Data traffic from source IP address 192.168.1.1 (behind Spoke A) to destination IP address 192.168.2.1 (behind Spoke B) triggers Spoke A to send a resolution request for Spoke B (10.0.0.12) to the next hop router (hub).
    • The hub receives the resolution request and finds a mapping entry for Spoke B (10.0.0.12). Because Spoke B is behind a NAT device, it acts as a proxy and replies with its own NBMA address (172.17.0.1).
    • The hub also receives a resolution request from Spoke B for Spoke A (10.0.0.11). Because Spoke B is behind a NAT device, it acts as a proxy and replies with its own NBMA address (172.17.0.1). This restricts any spoke-to-spoke traffic to or from Spoke B to travel through the hub router, which is done rather than having a tunnel between the spokes.

NHRP Spoke-to-Spoke Tunnel with a NAT Device

The NHRP Spoke-to-Spoke Tunnel with NAT introduces NAT extension in the NHRP protocol and is enabled automatically. The NHRP NAT extension is a Client Information Entry (CIE) entry with information about the protocol and post-NAT NBMA address. This additional information allows the support of spoke-to-spoke tunnels between spokes where one or both are behind a NAT device without the problem of losing (black-holing) traffic for an extended period of time.

Note


The spoke-to-spoke tunnel may fail to come up, but it is detected and the data traffic flows through the hub, rather than being lost (black-holed).

the diagram below shows how the NHRP spoke-to-spoke tunnel works with NAT.
Figure 2. NHRP Between Spoke-to-Spoke Tunnels

NHRP Registration Process

The following steps describe the NHRP registration process:
  1. A spoke sends a registration request with the NAT-Capability=1 parameter and a NAT NHRP extension of the NBMA address of the hub as configured on the spoke.
  2. The hub compares the NHRP (NAT) extension with its configured NBMA address and determines whether it itself is or is not behind a NAT device. The hub also makes a note of whether the spoke is behind a NAT device by comparing the incoming GRE/IP source address with the spoke’s NBMA address in the NHRP packet.
  3. The registration reply from the hub to the spoke includes a NAT NHRP extension with the post-NAT address of the spoke, if the hub detects if it is behind a NAT device.
  4. If the spokes get a NAT NHRP extension in the NHRP registration reply it then records its post-NAT IP address for possible use later.

NHRP Resolution and Purge Process

The following steps describe the NHRP resolution and purge process:
  1. When a spoke is behind a NAT device, it includes a NAT NHRP extension when it sends NHRP resolution requests.
  2. The hub receives the resolution request. If the spoke is behind a NAT device and there is no NAT extension, then the hub adds a NAT extension before forwarding this extension to the next node (spoke or next hop server) along the path. However, if the hub is forwarding the request to a non-NAT extension capable node, it rewrites the source-NBMA inside the packet to be the post-NAT IP address for the requesting spoke rather than its pre-NAT IP address.
  3. The receiver (spoke) uses a NAT NHRP extension record (NAT capable) or the source NBMA address (non-NAT capable information) to build the tunnel. This spoke’s reply includes its own NAT extension if it is behind a NAT device.

Note


Hubs do not answer NHRP resolution requests on behalf of spokes. Hubs always forward NHRP resolution requests to the end spoke that has the requested tunnel IP address or services the requested data from the host IP address.

The following describes the NHRP resolution process between Spoke A and Spoke B shown in the figure above, where Spoke B is behind a NAT device with pre-NAT address 172.16.2.1 and post-NAT address of 172.18.2.1:
  • Data traffic to the 192.168.2.0/24 network from hosts behind Spoke A triggers an NHRP resolution request for Spoke B’s tunnel IP address (10.0.0.12) to be sent through the hub. The hub receives a resolution request and forwards it to Spoke B. Spoke B creates a dynamic spoke-to-spoke tunnel using the source NBMA IP address for Spoke A from the NHRP resolution request and sends an NHRP resolution reply directly to Spoke A. It includes its post-NAT address in the NAT NHRP-extension header.
  • Alternatively, traffic to the192.168.1.0/24 network from hosts behind the NAT device on Spoke B triggers an NHRP resolution request for Spoke A’s tunnel IP address (10.0.0.11). Spoke B adds its own post-NAT IP address in the NHRP NAT-extension in the resolution request. The hub receives a resolution request and forwards it to Spoke A. Spoke A parses the NHRP NAT-extension and builds a tunnel using Spoke B’s post-NAT address and replies directly to Spoke B.

DMVPN Phase 1 , 2 and 3


DMVPN stands for Dynamic Multipoint VPN and it is an effective solution for dynamic secure overlay networks. In short, DMVPN is combination of the following technologies:
1) Multipoint GRE (mGRE)
2) Next-Hop Resolution Protocol (NHRP)
4) Dynamic Routing Protocol (EIGRP, RIP, OSPF, BGP)
3) Dynamic IPsec encryption
5) Cisco Express Forwarding (CEF)

Assuming that we has a general understanding of what DMVPN is and a solid understanding of IPsec/CEF, we are going to describe the role and function of each component in details. In this post we are going to illustrate two major phases of DMVPN evolution:
1) Phase 1 – Hub and Spoke (mGRE hub, p2p GRE spokes)
2) Phase 2 – Hub and Spoke with Spoke-to-Spoke tunnels (mGRE everywhere)
As for DMVPN Phase 3 – “Scalable Infrastructure”, a separate post is required to cover the subject. This is due to the significant changes made to NHRP resolution logic (NHRP redirects and shortcuts), which are better being illustrated when a reader has good understanding of first two phases. However, some hints about Phase 3 will be also provided in this post.
Note: Before we start, I would like to thank my friend Alexander Kitaev, for taking time to review the post and providing me with useful feedback.

Multipoint GRE
Let us start with the most basic building component of DMVPN – multipoint GRE tunnel. Classic GRE tunnel is point-to-point, but mGRE generalizes this idea by allowing a tunnel to have “multiple” destinations.
GRE Tunnels
This may seem natural if the tunnel destination address is multicast (e.g. 239.1.1.1). The tunnel could be used to effectively distribute the same information (e.g. video stream) to multiple destinations on top of a multicast-enabled network. Actually, this is how mGRE is used for Multicast VPN implementation in Cisco IOS. However, if tunnel endpoints need to exchange unicast packets, special “glue” is needed to map tunnel IP addresses to “physical” or “real” IP addresses, used by endpoint routers. As we’ll see later, this glue is called NHRP.
mGRE Tunnel
Note, that if you source multiple mGRE tunnels off the same interface (e.g. Loopback0) of a single router, then GRE can use special “multiplexor” field the tunnel header to differentiate them. This field is known as “tunnel key” and you can define it under tunnel configuration. As a matter of fact, up to IOS 12.3(14)T or 12.3(11)T3 the use of “tunnel key” was mandatory – mGRE tunnel would not come up, until the key is configured. Since the mentioned versions, you may configure a tunnel without the key. There were two reasons to remove the requirement. First, hardware ASICs of 6500 and 7600 platforms do not support mGRE tunnel-key processing, and thus the optimal switching performance on those platforms is penalized when you configure the tunnel key. Second, as we’ll see later, DMVPN Phase 3 allows interoperation between different mGRE tunnels sharing the same NHRP network-id only when they have the same tunnel-key or have no tunnel-key at all (since this allows sending packets “between” tunnels).
Generic NHRP
Now let’s move to the component that makes DMVPN truly dynamic – NHRP. The protocol has been defined quite some time ago in RFC 2332 (year 1998) to create a routing optimization scheme inside NBMA (non-broadcast multiple-access) networks, such as ATM, Frame-Relay and SMDS (anybody remembers this one nowadays? :) The general idea was to use SVC (switched virtual circuits) to create temporary shortcuts in non-fully meshed NBMA cloud. Consider the following schematic illustration, where IP subnet 10.0.0.0/24 overlays partial-meshed NBMA cloud. NHRP is similar in function to ARP, allowing resolving L3 to L2 addresses, but does that in more efficient manner, suitable for partially meshed NBMA clouds supporting dynamic layer 2 connections.
NHRP Illustration
The following is simplified and schematic illustration of NHRP process. In the above topology, in order for R1 to reach R4, it must send packets over PVCs between R1-R2, R2-R3 and finally R3-R4. Suppose the NMBA cloud allows using SVC (Switched virtual circuits, dynamic paths) – then it would be more reasonable for R1 to establish SVC directly with R4 and send packets over the optimal way. However, this requires R1 to know NMBA address (e.g. ATM NSAP) associated with R4 to “place a call”. Preferably, it would be better to make R1 learn R4 IP address to NSAP (NBMA address) mapping dynamically.
Now assume we enable NHRP on all NBMA interfaces in the network. Each router in topology acts as either NHC (Next-Hop Client) or NHS (Next-Hop Server). One of the functions of NHC is to register with NHS its IP address mapped to NBMA Layer 2 address (e.g. ATM NSAP address). To make registration possible, you configure each NHC with the IP address of at least one NHS. In turn, NHS acts as a database agent, storing all registered mappings, and replying to NHC queries. If NHS does not have a requested entry in its database, it can forward packet to another NHS to see if it has the requested association. Note that a router may act as a Next-Hop server and client at the same time. Back to the diagram, assume that R2 and R3 are NHSes, R1 and R4 are NHCs. Further, assume R4 is NHC and registers its IP to NBMA address mapping with R4 and R1 thinks R2 is the NHS. Both R2 and R3 treat each other as NHS. When R1 wants to send traffic to R4 (next-hop 10.0.0.4), it tries to resolve 10.0.0.4 by sending NHRP resolution request to R2 – the configured NHS. In turn, R2 will forward request to R3, since it has no local information.
Obviously, modern networks tend not to use ATM/SMDS and Frame-Relay SVC too much, but one can adopt NHRP to work with “simulated NBMA” networks, such as mGRE tunnels. The NBMA layer maps to “physical” underlying network while mGRE VPN is the “logical” network (tunnel internal IP addressing). In this case, mGRE uses NHRP for mapping “logical” or “tunnel inside” IP addresses to “physical” or real IP addresses. Effectively, NHRP perform the “glue” function described above, allowing mGRE endpoints discovering each other’s real IP address. Since NHRP defines a server role, it’s natural to have mGRE topology lay out in Hub-and-Spoke manner (or combination of hubs and spokes, in more advanced cases). Let’s see some particular scenarios to illustrate NHRP functionality with mGRE.
NHRP Phase 1
With NHRP Phase 1 mGRE uses NHRP to inform the hub about dynamically appearing spokes. Initially, you configure every spoke with the IP address of the hub as the NHS server. However, the spoke’s tunnel mode is GRE (regular point-to-point) tunnel with a fixed destination IP that equals to the physical address of the hub. The spokes can only reach the hub and only get to other spoke networks across the hub. The benefit of Phase 1 is simplified hub router configuration, which does not require static NHRP mapping for every new spoke.

As all packets go across the hub, almost any dynamic routing protocol would help with attaining reachability. The hub just needs to advertise a default route to spokes, while spokes should advertise their subnets dynamically to the hub. Probably it makes sense to run EIGRP and summarize all subnets to 0.0.0.0/0 on the hub, effectively sending a default route to all spokes (if the spokes do not use any other default route, e.g. from their ISPs). Configure spokes as EIGRP stubs and advertise their respective connected networks. RIP could be set up in similar manner, by simply configuring GRE tunnels on spokes as passive interfaces. Both EIGRP and RIP require split-horizon disabled on the hub mGRE interface in order to exchange subnets spoke to spoke. As for OSPF, the optimal choice would be using point-to-multipoint network type on all GRE and mGRE interfaces. In addition to that, configure ip ospf database filter-all out on the hub and set up static default routes via tunnel interfaces on the spokes (or static specific routes for corporate networks).
Here is a sample configuration. The detailed explanation of NHRP commands and “show” commands output follows the example.
mGRE + NHRP Phase 1 + EIGRP
R1:
!
! Hub router
!
router eigrp 123
 no auto-summary
 network 10.0.0.0 0.255.255.255
!
! Tunnel source
!
interface Loopback0
 ip address 150.1.1.1 255.255.255.0
!
! VPN network
!
interface Loopback 1
 ip address 10.0.1.1 255.255.255.0
!
! mGRE tunnel
!
interface Tunnel0
 ip address 10.0.0.1 255.255.255.0
 no ip redirects
 ip nhrp authentication cisco
 ip nhrp map multicast dynamic
 ip nhrp network-id 123
 no ip split-horizon eigrp 123
 ip summary-address eigrp 123 0.0.0.0 0.0.0.0 5
 tunnel source Loopback0
 tunnel mode gre multipoint
 tunnel key 123

R2:
!
! Spoke Router
!
router eigrp 123
 no auto-summary
 network 10.0.0.0 0.255.255.255
 eigrp stub connected
!
interface Loopback0
 ip address 150.1.2.2 255.255.255.0
!
interface Loopback 1
 ip address 10.0.2.2 255.255.255.0
!
! GRE tunnel
!
interface Tunnel0
 ip address 10.0.0.2 255.255.255.0
 ip nhrp authentication cisco
 ip nhrp map multicast 150.1.1.1
 ip nhrp map 10.0.0.1 150.1.1.1
 ip nhrp nhs 10.0.0.1
 ip nhrp network-id 123
 ip nhrp registration timeout 30
 ip nhrp holdtime 60
 tunnel source Loopback0
 tunnel destination 150.1.1.1
 tunnel key 123

R3:
!
! Spoke Router
!
router eigrp 123
 no auto-summary
 network 10.0.0.0 0.255.255.255
 eigrp stub connected
!
interface Loopback0
 ip address 150.1.3.3 255.255.255.0
!
interface Loopback 1
 ip address 10.0.3.3 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.3 255.255.255.0
 ip nhrp authentication cisco
 ip nhrp map multicast 150.1.1.1
 ip nhrp map 10.0.0.1 150.1.1.1
 ip nhrp nhs 10.0.0.1
 ip nhrp network-id 123
 ip nhrp registration timeout 30
 ip nhrp holdtime 60
 tunnel source Loopback0
 tunnel destination 150.1.1.1
 tunnel key 123
Note that only the hub tunnel uses mGRE encapsulation, and spokes use regular point-to-point GRE tunnels. Now, let’s look at the NHRP commands used in the example above. The most basic command ip nhrp map [Logical IP] [NBMA IP] – creates a static binding between a logical IP address and NBMA IP address. Since mGRE is treated by NHRP as NMBA medium, logical IP corresponds to the IP address “inside” a tunnel (“inner”) and the NBMA IP address corresponds to the “outer” IP address (the IP address used to source a tunnel). (From now on, we are going to call “inner” IP address and simply “IP address” or “logical IP address” and the “outer” IP address as “NBMA address” or “physical IP address”). The use of static NHRP mappings is to “bootstrap” information for the spokes to reach the logical IP address of the hub. The next command is ip nhrp map multicast dynamic|[StaticIP] and its purpose is the same as “frame-relay map… broadcast”. The command specifies the list of destination that will receive the multicast/broadcast traffic originated from this router. Spokes map multicasts to the static NBMA IP address of the hub, but hub maps multicast packets to the “dynamic” mappings – that is, the hub replicates multicast packets to all spokes registered via NHRP. Mapping multicasts is important in order to make dynamic routing protocol establish adjacencies and exchange update packets. The ip nhrp nhs [ServerIP] command configures NHRP client with the IP address of its NHRP server. Note the “ServerIP” is the logical IP address of the hub (inside the tunnel) and therefore spokes need the static NHRP mappings in order to reach it. The spokes use the NHS to register their logical IP to NBMA IP associations and send NHRP resolution request. (However, in this particular scenarios, the spokes will not send any NHRP Resolutions Requests, since they use directed GRE tunnels – only registration requests will be sent). The commands ip nhrp network-id and ip nhrp authentication [Key] identify and authenticate the logical NHRP network. The [ID] and the [Key] must match on all routers sharing the same GRE tunnel. It is possible to split an NBMA medium into multiple NHRP networks, but this is for advanced scenarios. As for the authentication, it’s a simple plain-text key sent in all NHRP messages. While the “network-id” is mandatory in order for NHRP to work, you may omit the authentication. Next command is ip nhrp holdtime that specifies the hold-time value set in NHRP registration requests. The NHS will keep the registration request cached for the duration of the hold-time, and then, if no registration update is receive, will time it out. The NHS will also send the same hold-time in NHRP resolution responses, if queried for the respective NHRP association. Note that you configure the ip nhrp holdtime command on spokes, and spoke will send registration requests every 1/3 of the hold-time seconds. However, if you also configure the ip nhrp registration timeout [Timeout] on a spoke, the NHRP registration requests will be sent every [Timeout] sends, not 1/3 of the configured hold-time. The hold-time value sent in NHRP Registration Requests will remain the same, of course.
Now let’s move to the show commands. Since it’s only the hub that uses the NHRP dynamic mappings to resolve the spokes NBMA addresses, it is useful to observe R1 NHRP cache:
Rack1R1#show ip nhrp detail
10.0.0.2/32 via 10.0.0.2, Tunnel0 created 00:16:59, expire 00:00:30
  Type: dynamic, Flags: authoritative unique registered used
  NBMA address: 150.1.2.2
10.0.0.3/32 via 10.0.0.3, Tunnel0 created 00:11:34, expire 00:00:55
  Type: dynamic, Flags: authoritative unique registered used
  NBMA address: 150.1.3.3
As we can see, the logical IP “10.0.0.2” maps to NBMA address “150.1.2.2” and the logical IP 10.0.0.3 maps to NBMA address 150.1.3.3. The “authoritative” flag means that the NHS has learned about the NHRP mapping directly from a registration request (the NHS “serves” the particular NHC). The “unique” flag means that the NHRP registration request had the same “unique” flag set. The use of this flag is to prevent duplicate NHRP mappings in cache. If unique mapping for a particular logical IP is already in the NHRP cache and another NHC tries to register the same logical IP with the NHS, the server will reject the registration, until the unique entry expires. Note that by default IOS routers set this flag in registration request, and this can be disabled by using ip nhrp registration no-unique command on a spoke. Sometimes this may be needed when spoke change its NBMA IP address often and needs to re-register a new mapping with the hub. The last flag, called “used” flag, means that the router uses the NHRP entry to switch IP packets. We will discuss the meaning of this flag in NRHP process switching section below. Also, note the “expires” field, which is a countdown timer, started from the “holdtime” specified in the Registration Request packet.
Let’s see the NHRP registration and reply process flows on the NHS.
Rack1R1#debug nhrp
NHRP protocol debugging is on

Rack1R1#debug nhrp packet
NHRP activity debugging is on
First, R3 tries to register its Logical IP to NBMA IP mapping with the hub. Note the specific NHRP packet format, split in three parts.
1) (F) – fixed part. Specifies the version, address family (afn) and protocol type (type) for resolution, as well as subnetwork layer (NBMA) type and length (shtl and sstl). Note that “shtl” equals 4, which is the length of IPv4 address in bytes, and “sstl” is for “subaddress” field which is not used with IPv4.
2) (M) – mandatory header part. Specifies some flags, like “unique” flag and the “Request ID”, which is used to track request/responses. Also includes are the source NBMA address (tunnel source in GRE/mGRE) and the source/destination protocol IP addresses. Destination IP address is the logical IP address of the hub and the source IP address is the logical IP address of the spoke. Using this information hub may populate the spoke logical IP address to NBMA IP address mapping.
3) (C-1) – CIE 1, which stands for “Client Information Element” field. While it’s not used in the packets below, in more advanced scenarios explored later, we’ll see this filed containing the information about networks connected to requesting/responding routers.
Also note the NAT-check output, which is Cisco’s extension used to make NHRP work for routers that tunnel from behind the NAT.
NHRP: Receive Registration Request via Tunnel0 vrf 0, packet size: 81
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "unique", reqid: 26
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.1
 (C-1) code: no error(0)
       prefix: 255, mtu: 1514, hd_time: 60
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 1
NHRP: NAT-check: matched destination address 150.1.3.3
NHRP: Tu0: Found and skipping dynamic multicast mapping  NBMA: 150.1.3.3
NHRP: Attempting to send packet via DEST 10.0.0.3
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.3.3
After processing the request, the router responds with NHRP Registration Reply. Note that the (M) header did not change, just the source and destination logical IP address of the packet are reversed. (R1->R3)
NHRP: Send Registration Reply via Tunnel0 vrf 0, packet size: 101
 src: 10.0.0.1, dst: 10.0.0.3
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "unique", reqid: 26
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.1
 (C-1) code: no error(0)
       prefix: 255, mtu: 1514, hd_time: 60
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 101 bytes out Tunnel0
Now the NHS receives the Registration Request from R2, and adds the corresponding entry in its NHRP cache
NHRP: Receive Registration Request via Tunnel0 vrf 0, packet size: 81
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "unique", reqid: 38
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.0.1
 (C-1) code: no error(0)
       prefix: 255, mtu: 1514, hd_time: 60
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 1
NHRP: NAT-check: matched destination address 150.1.2.2
NHRP: Tu0: Found and skipping dynamic multicast mapping  NBMA: 150.1.2.2
NHRP: Attempting to send packet via DEST 10.0.0.2
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.2.2

NHRP: Send Registration Reply via Tunnel0  vrf 0, packet size: 101
 src: 10.0.0.1, dst: 10.0.0.2
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "unique", reqid: 38
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.0.1
 (C-1) code: no error(0)
       prefix: 255, mtu: 1514, hd_time: 60
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 101 bytes out Tunnel0
 
We see how NRHP Phase 1 works now. The spokes register their associations with the hub via NHRP and the hub learns their NBMA addresses dynamically. At the same time, spokes use point-to-point tunnels to speak to the hub and reach each other. Note that EIGRP is not the only protocol suitable for use with NHRP Phase 1. OSPF is also a viable solution, thank to point-to-multipoint network type and database filter-all out command. See the example below for OSPF configuration with NHRP Phase 1:
mGRE + NHRP Phase 1 + OSPF
R1:
!
! Hub router
!
router ospf 123
 router-id 10.0.0.1
 network 10.0.0.0 0.255.255.255 area 0
!
interface Loopback0
 ip address 150.1.1.1 255.255.255.0
!
interface Loopback 1
 ip address 10.0.1.1 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.1 255.255.255.0
 no ip redirects
 ip nhrp authentication cisco
 ip nhrp map multicast dynamic
 ip nhrp network-id 123
 tunnel source Loopback0
 tunnel mode gre multipoint
 tunnel key 123
 ip ospf network point-to-multipoint
 ip ospf database-filter all out

R2:
!
! Spoke Router
!
router ospf 123
 network 10.0.0.0 0.255.255.255 area 0
 router-id 10.0.0.2
!
interface Loopback0
 ip address 150.1.2.2 255.255.255.0
!
interface Loopback 1
 ip address 10.0.2.2 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.2 255.255.255.0
 ip nhrp authentication cisco
 ip nhrp map multicast 150.1.1.1
 ip nhrp map 10.0.0.1 150.1.1.1
 ip nhrp nhs 10.0.0.1
 ip nhrp network-id 123
 ip nhrp registration timeout 30
 ip nhrp holdtime 60
 tunnel source Loopback0
 tunnel destination 150.1.1.1
 tunnel key 123
 ip ospf network point-to-multipoint
!
ip route 0.0.0.0 0.0.0.0 Tunnel0

R3:
!
! Spoke Router
!
router ospf 123
 network 10.0.0.0 0.255.255.255 area 0
 router-id 10.0.0.3
!
interface Loopback0
 ip address 150.1.3.3 255.255.255.0
!
interface Loopback 1
 ip address 10.0.3.3 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.3 255.255.255.0
 ip nhrp authentication cisco
 ip nhrp map multicast 150.1.1.1
 ip nhrp map 10.0.0.1 150.1.1.1
 ip nhrp nhs 10.0.0.1
 ip nhrp network-id 123
 ip nhrp registration timeout 30
 ip nhrp holdtime 60
 tunnel source Loopback0
 tunnel destination 150.1.1.1
 tunnel key 123
 ip ospf network point-to-multipoint
!
ip route 0.0.0.0 0.0.0.0 Tunnel0
As we said, the main benefit of using NHRP Phase 1 is simplified configuration on the hub router. Additionally, spoke routers receive minimal routing information (it’s either summarized or filtered on the hub) and are configured in uniform manner. In most simple case, spoke routers could be configured without any NHRP, by simply using point-to-point GRE tunnels. This scenario requires the hub to create a static NHRP mapping for every spoke. For example:
mGRE + NHRP Phase 1 + OSPF + Static NHRP mappings
R1:
!
! Hub router
!
router ospf 123
 router-id 10.0.0.1
 network 10.0.0.0 0.255.255.255 area 0
!
interface Loopback0
 ip address 150.1.1.1 255.255.255.0
!
interface Loopback 1
 ip address 10.0.1.1 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.1 255.255.255.0
 no ip redirects
 ip nhrp authentication cisco
 ip nhrp map 10.0.0.2 150.1.2.2
 ip nhrp map 10.0.0.3 150.1.3.3
 ip nhrp map multicast 150.1.2.2
 ip nhrp map multicast 150.1.3.3
 ip nhrp network-id 123
 tunnel source Loopback0
 tunnel mode gre multipoint
 tunnel key 123
 ip ospf network point-to-multipoint
 ip ospf database-filter all out

R2:
!
! Spoke Router
!
router ospf 123
 network 10.0.0.0 0.255.255.255 area 0
 router-id 10.0.0.2
!
interface Loopback0
 ip address 150.1.2.2 255.255.255.0
!
interface Loopback 1
 ip address 10.0.2.2 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.2 255.255.255.0
 tunnel source Loopback0
 tunnel destination 150.1.1.1
 tunnel key 123
 ip ospf network point-to-multipoint
!
ip route 0.0.0.0 0.0.0.0 Tunnel0

R3:
!
! Spoke Router
!
router ospf 123
 network 10.0.0.0 0.255.255.255 area 0
 router-id 10.0.0.3
!
interface Loopback0
 ip address 150.1.3.3 255.255.255.0
!
interface Loopback 1
 ip address 10.0.3.3 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.3 255.255.255.0
 tunnel source Loopback0
 tunnel destination 150.1.1.1
 tunnel key 123
 ip ospf network point-to-multipoint
!
ip route 0.0.0.0 0.0.0.0 Tunnel0
The disadvantage of NHRP Phase 1 is the inability to establish spoke-to-spoke shortcut tunnels. NHRP Phase 2 resolves this issue and allows for spoke-to-spoke tunnels. To better understand the second phase, we first need to find out how NHRP interacts with CEF – the now default IP switching method on most Cisco routers. Consider the topology and example configuration that follows. See the detailed breakdown after the configuration.
mGRE + NHRP Phase 2 + EIGRP
DMPVN Phase 2
R1:
!
! Hub router
!
router eigrp 123
 no auto-summary
 network 10.0.0.0 0.255.255.255
!
interface Loopback0
 ip address 150.1.1.1 255.255.255.0
!
interface Loopback 1
 ip address 10.0.1.1 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.1 255.255.255.0
 no ip redirects
 ip nhrp authentication cisco
 ip nhrp map multicast dynamic
 ip nhrp network-id 123
 no ip split-horizon eigrp 123
 no ip next-hop-self eigrp 123
 tunnel source Loopback0
 tunnel mode gre multipoint
 tunnel key 123

R2:
!
! Spoke Router
!
router eigrp 123
 no auto-summary
 network 10.0.0.0 0.255.255.255
 eigrp stub connected
!
interface Loopback0
 ip address 150.1.2.2 255.255.255.0
!
interface Loopback 1
 ip address 10.0.2.2 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.2 255.255.255.0
 ip nhrp authentication cisco
 ip nhrp map multicast 150.1.1.1
 ip nhrp map 10.0.0.1 150.1.1.1
 ip nhrp nhs 10.0.0.1
 ip nhrp network-id 123
 ip nhrp registration timeout 30
 ip nhrp holdtime 60
 tunnel source Loopback0
 tunnel mode gre multipoint
 tunnel key 123

R3:
!
! Spoke Router
!
router eigrp 123
 no auto-summary
 network 10.0.0.0 0.255.255.255
 eigrp stub connected
!
interface Loopback0
 ip address 150.1.3.3 255.255.255.0
!
interface Loopback 1
 ip address 10.0.3.3 255.255.255.0
!
interface Tunnel0
 ip address 10.0.0.3 255.255.255.0
 ip nhrp authentication cisco
 ip nhrp map multicast 150.1.1.1
 ip nhrp map 10.0.0.1 150.1.1.1
 ip nhrp nhs 10.0.0.1
 ip nhrp network-id 123
 ip nhrp registration timeout 30
 ip nhrp holdtime 60
 tunnel source Loopback0
 tunnel mode gre multipoint
 tunnel key 123
Note that both spokes use mGRE tunnel encapsulation mode, and the hub sets the originating router next-hop IP address in “reflected” EIGRP updates (by default EIGRP sets the next-hop field to “0.0.0.0” – that is, to self). By the virtue of the EIGRP configuration, the subnet “10.0.2.0/24” (attached to R2) reaches to R3 with the next-hop IP address of “10.0.0.2” (R2). It is important that R3 learns “10.0.2.0/24” with the next hop of R2 logical IP address. As we see later, this is the key to trigger CEF next-hop resolution. The mGRE encapsulation used on spokes will trigger NHRP resolutions since now this is NBMA medium. Now, assuming that traffic to 10.0.2.0/24 does not flow yet, check the routing table entry for 10.0.2.2 and the CEF entries for the route and its next-hop:
Rack1R3#show ip route 10.0.2.2
Routing entry for 10.0.2.0/24
  Known via "eigrp 123", distance 90, metric 310172416, type internal
  Redistributing via eigrp 123
  Last update from 10.0.0.2 on Tunnel0, 00:09:55 ago
  Routing Descriptor Blocks:
  * 10.0.0.2, from 10.0.0.1, 00:09:55 ago, via Tunnel0
      Route metric is 310172416, traffic share count is 1
      Total delay is 1005000 microseconds, minimum bandwidth is 9 Kbit
      Reliability 255/255, minimum MTU 1472 bytes
      Loading 1/255, Hops 2

Rack1R3#show ip cef 10.0.2.2
10.0.2.0/24, version 48, epoch 0
0 packets, 0 bytes
  via 10.0.0.2, Tunnel0, 0 dependencies
    next hop 10.0.0.2, Tunnel0
    invalid adjacency

Rack1R3#show ip cef 10.0.0.2
10.0.0.0/24, version 50, epoch 0, attached, connected
0 packets, 0 bytes
  via Tunnel0, 0 dependencies
    valid glean adjacency
Note that CEF prefix for “10.0.2.0/24” is invalid (but not “glean”), since “10.0.0.2” has not yet been resolved. The CEF prefix for “10.0.0.2” has “glean” adjacency, which means the router needs to send an NHRP resolution request to map the logical IP to NBMA address. Therefore, with CEF switching, NHRP resolution requests are only sent for “next-hop” IP addresses, and never for the networks (e.g. 10.0.2.0/24) themselves (the process-switching does resolve any prefix as we’ll see later). Go ahead and ping from R3 to “10.0.3.3” and observe the process:
Rack1R3#ping 10.0.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/80/180 ms
Check the mappings on the hub router. The only two entries registered are the VPN IP addresses of R2 and R3, together with the respective NBMA IP addresses. Note the “expire” field, which, as mentioned above, counts the time for the entry to expire based on the “holdtime” settings of the registering router’s interface. Later we will see how CEF uses this countdown timer to refresh or delete CEF entries for the next-hop IP address
Rack1R1#show ip nhrp
10.0.0.2/32 via 10.0.0.2, Tunnel0 created 00:16:33, expire 00:00:43
  Type: dynamic, Flags: authoritative unique registered
  NBMA address: 150.1.2.2
  Requester: 10.0.0.3 Request ID: 798
10.0.0.3/32 via 10.0.0.3, Tunnel0 created 00:16:34, expire 00:00:51
  Type: dynamic, Flags: authoritative unique registered
  NBMA address: 150.1.3.3
  Requester: 10.0.0.2 Request ID: 813

Check the mappings on R2 (note that R2 now has mapping for R3’s
next-hop associated with its NBMA IP address) 

Rack1R2#show ip nhrp
10.0.0.1/32 via 10.0.0.1, Tunnel0 created 00:14:52, never expire
  Type: static, Flags: authoritative used
  NBMA address: 150.1.1.1
10.0.0.2/32 via 10.0.0.2, Tunnel0 created 00:05:49, expire 00:00:10
  Type: dynamic, Flags: router authoritative unique local
  NBMA address: 150.1.2.2
    (no-socket)
10.0.0.3/32 via 10.0.0.3, Tunnel0 created 00:00:30, expire 00:00:29
  Type: dynamic, Flags: router used
  NBMA address: 150.1.3.3 

The same command output on R3 is symmetric to the output on R2:

Rack1R3#show ip nhrp
10.0.0.1/32 via 10.0.0.1, Tunnel0 created 00:14:00, never expire
  Type: static, Flags: authoritative used
  NBMA address: 150.1.1.1
10.0.0.2/32 via 10.0.0.2, Tunnel0 created 00:00:05, expire 00:00:54
  Type: dynamic, Flags: router
  NBMA address: 150.1.2.2
10.0.0.3/32 via 10.0.0.3, Tunnel0 created 00:01:46, expire 00:00:13
  Type: dynamic, Flags: router authoritative unique local
  NBMA address: 150.1.3.3
    (no-socket)
Now check the CEF entry for R2’s next-hop IP address on R3:
Rack1R3#sh ip cef 10.0.0.2
10.0.0.2/32, version 65, epoch 0, connected
0 packets, 0 bytes
  via 10.0.0.2, Tunnel0, 0 dependencies
    next hop 10.0.0.2, Tunnel0
    valid adjacency
The CEF entry for “10.0.0.2” is now valid, since NHRP mapping entry is present. If the next-hop for the prefix “10.0.2.0/24” was pointing toward the hub (R1) (e.g. if the hub was using the default ip next-hop-self eigrp 123) then the NHRP lookup will not be triggered, and cut-through NHRP entry will not be installed. Let’s see the debugging command output on R1, R2 and R3 to observe how the routers collectively resolve the next-hop IP addresses when R3 pings R1:
Rack1R1#debug nhrp
NHRP protocol debugging is on
Rack1R1#debug nhrp packet
NHRP activity debugging is on

Rack1R2#debug nhrp
NHRP protocol debugging is on
Rack1R2#debug nhrp packet
NHRP activity debugging is on

Rack1R3#debug nhrp
NHRP protocol debugging is on
Rack1R3#debug nhrp packet
NHRP activity debugging is on
It all starts when R3 tries to route a packet to “10.0.2.2” and finds out it has “glean” adjacency for its next-hop of “10.0.0.2”. Then, R3 attempt to send NHRP resolution request directly to R2, but fails since R2 NMBA address is unknown. At the same time, the original data packet (ICMP echo) follows to R2 across the hub (R1).
Rack1R3#
NHRP: MACADDR: if_in null netid-in 0 if_out Tunnel0 netid-out 123
NHRP: Checking for delayed event 0.0.0.0/10.0.0.2 on list (Tunnel0).
NHRP: No node found.
NHRP: Sending packet to NHS 10.0.0.1 on Tunnel0
NHRP: Checking for delayed event 0.0.0.0/10.0.0.2 on list (Tunnel0).
NHRP: No node found.
NHRP: Attempting to send packet via DEST 10.0.0.2
NHRP: Send Resolution Request via Tunnel0 vrf 0, packet size: 81
 src: 10.0.0.3, dst: 10.0.0.2
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 994
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: Encapsulation failed for destination 10.0.0.2 out Tunnel0
Next, R3 tries to send resolution request to the NHS, which is R1. The resolution request contains information about source NBMA address of R3, and source protocol (logical IP) addresses of R3 and R2.
Rack1R3#
NHRP: Attempting to send packet via NHS 10.0.0.1
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.1.1
NHRP: Send Resolution Request via Tunnel0 vrf 0, packet size: 81
 src: 10.0.0.3, dst: 10.0.0.1
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 994
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 81 bytes out Tunnel0
The Resolution Request from R3 arrives to NHS. In essence, R3 tries to resolve the “glean” CEF adjacency using NHRP the same way it uses ARP on Ethernet. Note that request only mentions logical IP addresses of R3 (“10.0.0.3”) and R2 (“10.0.0.2”) and NBMA address of R3.
Rack1R1#
NHRP: Receive Resolution Request via Tunnel0 vrf 0, packet size: 81
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 994
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 0
NHRP: NAT-check: matched destination address 150.1.3.3
NHRP: nhrp_rtlookup yielded Tunnel0
NHRP: Tu0: Found and skipping dynamic multicast mapping  NBMA: 150.1.3.3
NHRP: netid_out 123, netid_in 123
NHRP: nhrp_cache_lookup_comp returned 0x855C7B90
NHRP: Attempting to send packet via DEST 10.0.0.3
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.3.3
The NHS has the NHRP mapping for “10.0.0.2” in its NHRP cache – R2 registered this associating with R1. The NHS may immediately reply to the client. Note the “(C-1)” – CIE header in the NHRP reply packet. While the “(M)” (mandatory) header contains the same information received in request packet from R3, the CIE header contains the actual NHRP reply, with the mapping information for R2. This is because the NHS considers R2 to be the “client” of it, and therefore it sends the actual information in CIE header. Note the “prefix” length of 32 – this means the reply is just for one host logical IP address.
Rack1R1#
NHRP: Send Resolution Reply via Tunnel0 vrf 0, packet size: 109
 src: 10.0.0.1, dst: 10.0.0.3
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 994
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 32, mtu: 1514, hd_time: 342
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.2.2
       client protocol: 10.0.0.2
NHRP: 109 bytes out Tunnel0
At this point, R2 receives the original data packet from R3 (ICMP echo) and tries to send a response back. The problem is that the destination IP address for the echo reply is “10.0.3.3” and the next-hop is “10.0.0.3”, which has “glean” CEF adjacency. Again, R2 replies back across the hub and send a Resolution Request packet: first, directly R3 – this attempt fails – then it sends the resolution request to the NHS.
Rack1R2#
NHRP: MACADDR: if_in null netid-in 0 if_out Tunnel0 netid-out 123
NHRP: Checking for delayed event 0.0.0.0/10.0.0.3 on list (Tunnel0).
NHRP: No node found.
NHRP: Sending packet to NHS 10.0.0.1 on Tunnel0
NHRP: Checking for delayed event 0.0.0.0/10.0.0.3 on list (Tunnel0).
NHRP: No node found.
NHRP: Attempting to send packet via DEST 10.0.0.3
NHRP: Send Resolution Request via Tunnel0 vrf 0, packet size: 81
 src: 10.0.0.2, dst: 10.0.0.3
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 1012
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.0.3
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: Encapsulation failed for destination 10.0.0.3 out Tunnel0
NHRP: Attempting to send packet via NHS 10.0.0.1
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.1.1

Rack1R2#
NHRP: Send Resolution Request via Tunnel0 vrf 0, packet size: 81
 src: 10.0.0.2, dst: 10.0.0.1
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 1012
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.0.3
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 81 bytes out Tunnel0
NHRP: MACADDR: if_in null netid-in 0 if_out Tunnel0 netid-out 123
NHRP: Checking for delayed event 0.0.0.0/10.0.0.3 on list (Tunnel0).
NHRP: No node found.
NHRP: Sending packet to NHS 10.0.0.1 on Tunnel0
R3 finally receive the Resolution Reply from the NHS, and now it may complete the CEF adjacency for “10.0.0.2”. Since that moment, it switches all packets to “10.0.2.2” directly via R2, not across R1.
Rack1R3#
NHRP: Receive Resolution Reply via Tunnel0 vrf 0, packet size: 109
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 994
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 32, mtu: 1514, hd_time: 342
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.2.2
       client protocol: 10.0.0.2
NHRP: netid_in = 0, to_us = 1
NHRP: Checking for delayed event 150.1.2.2/10.0.0.2 on list (Tunnel0).
NHRP: No node found.
NHRP: No need to delay processing of resolution event nbma src:150.1.3.3 nbma dst:150.1.2.2
The resolution request that R2 sent before in attempted to resolve the NBMA address for “10.0.0.3” arrives to R1. Since the NHS has all the information in its local cache (R3 registered its IP to NBMA address mapping) it immediately replies to R2. Note the CIE header in the NHRP reply packet, which contains the actual mapping information.
Rack1R1#
NHRP: Receive Resolution Request via Tunnel0 vrf 0, packet size: 81
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 1012
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.0.3
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 0
NHRP: NAT-check: matched destination address 150.1.2.2
NHRP: nhrp_rtlookup yielded Tunnel0
NHRP: Tu0: Found and skipping dynamic multicast mapping  NBMA: 150.1.2.2
NHRP: netid_out 123, netid_in 123
NHRP: nhrp_cache_lookup_comp returned 0x848EF9E8
NHRP: Attempting to send packet via DEST 10.0.0.2
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.2.2

Rack1R1#
NHRP: Send Resolution Reply via Tunnel0 vrf 0, packet size: 109
 src: 10.0.0.1, dst: 10.0.0.2
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 1012
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.0.3
 (C-1) code: no error(0)
       prefix: 32, mtu: 1514, hd_time: 242
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.3.3
       bclient protocol: 10.0.0.3
NHRP: 109 bytes out Tunnel0
At last, R2 receive the reply to its original request, and now it has all the information to complete the CEF entry for “10.0.0.3” and switch packets across the optimal path to R3. At this moment both spokes have symmetric information to reach each other
Rack1R2#
NHRP: Receive Resolution Reply via Tunnel0 vrf 0, packet size: 109
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 1012
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.0.3
 (C-1) code: no error(0)
       prefix: 32, mtu: 1514, hd_time: 242
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.3.3
       client protocol: 10.0.0.3
NHRP: netid_in = 0, to_us = 1
NHRP: Checking for delayed event 150.1.3.3/10.0.0.3 on list (Tunnel0).
NHRP: No node found.
NHRP: No need to delay processing of resolution event nbma src:150.1.2.2 nbma dst:150.1.3.3
Timing out NHRP entries
Now that we know that CEF resolves the next-hop information via NHRP, how does it time-out the unused cut-through tunnel? As we remember, each NHRP entry has countdown expire timer, initialized from the registration hold-time. Every 60 seconds global NHRP process runs on a router and checks the expire timer on all NHRP entries. If the expire timer for an NHRP entry is greater than 120 seconds, nothing is done to the corresponding CEF entry. If the timer is less than 120 seconds, the NHRP process marks the corresponding CEF entry as “stale” but still usable. As soon as the router switches an IP packet using the “stale” entry, it triggers new NHRP resolution request, and eventually refreshes the corresponding NHRP entry as well as CEF entry itself. If no packet hits the “stale” CEF entry, the NHRP mapping will eventually time-out (since the router does not send any “refreshing” requests) and the corresponding CEF entry will become invalid. This will effectively tear down the spoke-to-spoke tunnel.
NHRP Phase 2 Conclusions
Let us quickly recap what we learned so far about NHRP Phase 2 and CEF. Firstly, this mode requires all the spokes to have complete routing information with the next-hop preserved. This may limit scalability in large networks, since not all spokes may accept full load of routing updates. Secondly, CEF only resolve the next-hop information via NHRP, not the full routing prefixes. Actually, the second feature directly implies the first limitation. As we noted, the no ip next-hop-self eigrp 123 command is required to make spoke-to-spoke tunnels work with CEF. However, they added the command only in IOS version 12.3. Is there a way to make spoke-to-spoke tunnels work when the next-hop is set to “self” (the default) in EIGRP updates? Actually, there are few ways. First and the best one – do not use old IOS images to implement DMVPN :) Actually, it is better to use the latest 12.4T train images with DMVPN Phase 3 for the deployment – but then again those images are from the “T”-train! OK, so the other option is get rid of EIGRP and use OSPF, with the network type “broadcast”. OSPF is a link-state protocol – it does not hide topology information and does not mask the next-hop in any way (well, at least when the network-type is “broadcast”). However, the limitation is that the corresponding OSPF topology may have just two redundant hubs – corresponding to OSPF DR and BDR for a segment. This is because every hub must form OSPF adjacencies with all spokes. Such limitation is not acceptable in large installations, but still works fine in smaller deployments. However, there is one final workaround, which is probably the one you may want to use in the current CCIE lab exam – disable CEF on spokes. This is a very interesting case per se, and we are going to see now NHRP works with process switching.
NHRP Phase 2 + EIGRP next-hop-self + no CEF
In this scenario, EIGRP next-hop self is enabled on R1 (the hub). Now R3 sees 10.0.2.0/24 with the next hop of R1. Disable CEF on R2 and R3, and try pinging 10.0.2.2 off R3 loopback1 interface.
R3 sees the route behind R2 as reachable via R1

Rack1R3#show ip route 10.0.2.2
Routing entry for 10.0.2.0/24
  Known via "eigrp 123", distance 90, metric 310172416, type internal
  Redistributing via eigrp 123
  Last update from 10.0.0.1 on Tunnel0, 00:09:55 ago
  Routing Descriptor Blocks:
  * 10.0.0.1, from 10.0.0.1, 00:09:55 ago, via Tunnel0
      Route metric is 310172416, traffic share count is 1
      Total delay is 1005000 microseconds, minimum bandwidth is 9 Kbit
      Reliability 255/255, minimum MTU 1472 bytes
      Loading 1/255, Hops 2
R3 pings “10.0.2.2”, sourcing packet off “10.0.3.3”. Since CEF is disabled, the system performs NHRP lookup to find the NBMA address for “10.0.2.2”. This is opposed to CEF behavior that would only resolve the next-hop for “10.0.2.2″ entry. Naturally, the router forwards NHRP request to R3’s NHS, which is R1. At the same time, R3 forwards the data packet (ICMP echo) via its current next-hop – “10.0.0.1”, that is via the hub.
Rack1R3#
NHRP: MACADDR: if_in null netid-in 0 if_out Tunnel0 netid-out 123
NHRP: Checking for delayed event 0.0.0.0/10.0.2.2 on list (Tunnel0).
NHRP: No node found.
NHRP: Sending packet to NHS 10.0.0.1 on Tunnel0
NHRP: Checking for delayed event 0.0.0.0/10.0.2.2 on list (Tunnel0).
NHRP: No node found.
NHRP: Attempting to send packet via DEST 10.0.2.2
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.1.1
NHRP: Send Resolution Request via Tunnel0 vrf 0, packet size: 81
 src: 10.0.0.3, dst: 10.0.2.2
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 900
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.2.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 81 bytes out Tunnel0
NHRP: MACADDR: if_in null netid-in 0 if_out Tunnel0 netid-out 123
NHRP: Checking for delayed event 0.0.0.0/10.0.2.2 on list (Tunnel0).
NHRP: No node found.
NHRP: Sending packet to NHS 10.0.0.1 on Tunnel0
Resolution Request arrives to R1 (the NHS). Since R1 has no mapping for “10.0.2.2” (R2 only registers the IP address 10.0.0.2 – its own next-hop IP address), the NHS looks up into routing table, to find the next-hop towards 10.0.2.2. Since it happens to be R2’s IP “10.0.0.2”, the NHS then tries to forward the resolution request towards the next router on the path to the network requested in resolution message – to R2. Thanks to R2’s NHRP registration with R1, the NHS now knows R2’s NBMA address, and successfully encapsulates the packet. In addition, R1 forwards the data packet from R1 to R2, using its routing table. Obviously, the data packet will arrive to R2 a little bit faster, since NHRP requires more time to process and forward the request.
Rack1R1#
NHRP: Receive Resolution Request via Tunnel0 vrf 0, packet size: 81
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 900
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.2.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 0
NHRP: NAT-check: matched destination address 150.1.3.3
NHRP: nhrp_rtlookup yielded Tunnel0
NHRP: Tu0: Found and skipping dynamic multicast mapping  NBMA: 150.1.3.3
NHRP: netid_out 123, netid_in 123
NHRP: nhrp_cache_lookup_comp returned 0x0
NHRP: Attempting to send packet via DEST 10.0.2.2
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.2.2

NHRP: Forwarding Resolution Request via Tunnel0 vrf 0, packet size: 101
 src: 10.0.0.1, dst: 10.0.2.2
 (F) afn: IPv4(1), type: IP(800), hop: 254, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 900
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.2.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 101 bytes out Tunnel0
Now the data packet (ICMP echo) has arrived to R2. R2 generates the response (ICMP – echo reply from “10.0.2.2” to “10.0.3.3”) and now R2 needs the NMBA address of “10.0.3.3” (CEF is disabled on R2). As usual, R2 generates a resolutions request to its NHS (R1). At the same time, R2 sends the response packet to R3’s request across the hub, since it does not know the NBMA address of R3.
Rack1R2#
NHRP: Send Resolution Request via Tunnel0 vrf 0, packet size: 81
 src: 10.0.0.2, dst: 10.0.3.3
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 919
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.3.3
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 81 bytes out Tunnel0
Soon after the data packet arrived, R2 receives the Resolution Request from R3 forwarded by R1. Since R2 is the egress router on NBMA segment for the network “10.0.2.2”, it may reply to the request.
Rack1R2#
NHRP: Receive Resolution Request via Tunnel0 vrf 0, packet size: 101
 (F) afn: IPv4(1), type: IP(800), hop: 254, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 900
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.2.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 0
NHRP: nhrp_rtlookup yielded Loopback1
NHRP: netid_out 0, netid_in 123
NHRP: We are egress router for target 10.0.2.2, recevied via Tunnel0
NHRP: Redist mask now 1
NHRP: Attempting to send packet via DEST 10.0.0.3
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.3.3
Note that R2 replies with the full prefix found in its routing table – “10.0.2.0/24”, not just single host “10.0.2.2/32” (this feature is critical for DMVPN Phase 3). This information is encapsulated inside “(C-1)” part of the NHRP reply packet (Client Information Element 1) which describes a client – network connected to the router (R2). The “prefix” field is “/24” which is exactly the value taken from the routing table.
Also note, that R2 learned R3’s NBMA address from the Resolution Request, and now replies directly to R3, bypassing R1. The “stable” flag means that the querying/replying router directly knows the source or destination IP address in the resolution request/reply.
Rack1R2#
NHRP: Send Resolution Reply via Tunnel0 vrf 0, packet size: 129
 src: 10.0.0.2, dst: 10.0.0.3   <-- NBMA addresses of R2/R3
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 900
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.2.2
 (C-1) code: no error(0)
       prefix: 24, mtu: 1514, hd_time: 360
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.2.2
       client protocol: 10.0.2.2
NHRP: 129 bytes out Tunnel0
At this moment, Resolution Request from R2 for network “10.0.3.3″ reaches R1 – the NHS. Since the NHS has no information on “10.0.3.3″, it forwards the request to R3 – the next-hop found via the routing table on path to “10.0.3.3″.
Rack1R1#
NHRP: Receive Resolution Request via Tunnel0 vrf 0, packet size: 81
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 919
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.3.3
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 0
NHRP: NAT-check: matched destination address 150.1.2.2
NHRP: nhrp_rtlookup yielded Tunnel0
NHRP: Tu0: Found and skipping dynamic multicast mapping  NBMA: 150.1.2.2
NHRP: netid_out 123, netid_in 123
NHRP: nhrp_cache_lookup_comp returned 0x0
NHRP: Attempting to send packet via DEST 10.0.3.3
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.3.3

NHRP: Forwarding Resolution Request via Tunnel0 vrf 0, packet size: 101
 src: 10.0.0.1, dst: 10.0.3.3
 (F) afn: IPv4(1), type: IP(800), hop: 254, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 919
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.3.3
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: 101 bytes out Tunnel0
Back to R3. At this point, it received the ICMP reply for the original ICMP echo packet. Now R3 receives the NHRP Resolution Reply to its original Resolution Request directly from R2. This allows R3 to learn that “10.0.2.0/24” is reachable via NMBA IP address “150.1.2.2”. Note that CIE field “(C-1)” in the reply packet, which tells R3 about the whole “10.0.2.0/24” network – the “prefix” is set to “24”.
Rack1R3#
NHRP: Receive Resolution Reply via Tunnel0 vrf 0, packet size: 129
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 900
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.2.2
 (C-1) code: no error(0)
       prefix: 24, mtu: 1514, hd_time: 360
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.2.2
       client protocol: 10.0.2.2
NHRP: netid_in = 0, to_us = 1
NHRP: NAT-check: matched destination address 150.1.2.2
NHRP: Checking for delayed event 150.1.2.2/10.0.2.2 on list (Tunnel0).
NHRP: No node found.
NHRP: No need to delay processing of resolution event nbma src:150.1.3.3 nbma dst:150.1.2.2
NHRP: Checking for delayed event 0.0.0.0/10.0.2.2 on list (Tunnel0).
NHRP: No node found.
Finally, the Resolution Request from R2, forwarded by R1 (the NHS) arrives to R3. The local router performs lookup for 10.0.3.3 and finds this to be directly connected network, with the prefix of /24. Therefore, R3 generates a Resolution Reply packet and sends it directly to R2, bypassing R1. This packet tells R2 to map logical IP “10.0.3.0/24” to NBMA address “150.1.3.3”.
Rack1R3#
NHRP: Receive Resolution Request via Tunnel0 vrf 0, packet size: 101
 (F) afn: IPv4(1), type: IP(800), hop: 254, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth src-stable", reqid: 919
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.3.3
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 360
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 0, pref: 0
NHRP: netid_in = 123, to_us = 0
NHRP: nhrp_rtlookup yielded Loopback1
NHRP: netid_out 0, netid_in 123
NHRP: We are egress router for target 10.0.3.3, recevied via Tunnel0
NHRP: Redist mask now 1
NHRP: Attempting to send packet via DEST 10.0.0.2
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.2.2

NHRP: Send Resolution Reply via Tunnel0 vrf 0, packet size: 129
 src: 10.0.0.3, dst: 10.0.0.2
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 919
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.3.3
 (C-1) code: no error(0)
       prefix: 24, mtu: 1514, hd_time: 360
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.3.3
       client protocol: 10.0.3.3
NHRP: 129 bytes out Tunnel0
At last, R2 receives the response to its Resolution Request, and everything is stable now. R2 and R3 know how to reach “10.0.3.0/24” and “10.0.2.0/24” respectively.
Rack1R2#
NHRP: Receive Resolution Reply via Tunnel0 vrf 0, packet size: 129
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "router auth dst-stable unique src-stable", reqid: 919
     src NBMA: 150.1.2.2
     src protocol: 10.0.0.2, dst protocol: 10.0.3.3
 (C-1) code: no error(0)
       prefix: 24, mtu: 1514, hd_time: 360
       addr_len: 4(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client NBMA: 150.1.3.3
       client protocol: 10.0.3.3
NHRP: netid_in = 0, to_us = 1
NHRP: NAT-check: matched destination address 150.1.3.3
NHRP: Checking for delayed event 150.1.3.3/10.0.3.3 on list (Tunnel0).
NHRP: No node found.
NHRP: No need to delay processing of resolution event nbma src:150.1.2.2 nbma dst:150.1.3.3
NHRP: Checking for delayed event 0.0.0.0/10.0.3.3 on list (Tunnel0).
NHRP: No node found.
Now let’s look at NHRP caches of all three routers:
Rack1R1#show ip nhrp
10.0.0.2/32 via 10.0.0.2, Tunnel0 created 01:00:47, expire 00:04:02
  Type: dynamic, Flags: authoritative unique registered
  NBMA address: 150.1.2.2
10.0.0.3/32 via 10.0.0.3, Tunnel0 created 01:00:47, expire 00:04:23
  Type: dynamic, Flags: authoritative unique registered
  NBMA address: 150.1.3.3 

Rack1R2#show ip nhrp
10.0.0.1/32 via 10.0.0.1, Tunnel0 created 01:56:30, never expire
  Type: static, Flags: authoritative used
  NBMA address: 150.1.1.1
10.0.0.3/32 via 10.0.0.3, Tunnel0 created 00:00:24, expire 00:05:35
  Type: dynamic, Flags: router implicit
  NBMA address: 150.1.3.3
  10.0.2.0/24 via 10.0.2.2, Tunnel0 created 00:00:24, expire 00:05:35
  Type: dynamic, Flags: router authoritative unique local
  NBMA address: 150.1.2.2
    (no-socket)
10.0.3.0/24 via 10.0.3.3, Tunnel0 created 00:00:24, expire 00:05:35
  Type: dynamic, Flags: router
  NBMA address: 150.1.3.3 

Rack1R3#show ip nhrp
10.0.0.1/32 via 10.0.0.1, Tunnel0 created 01:56:00, never expire
  Type: static, Flags: authoritative used
  NBMA address: 150.1.1.1
10.0.0.2/32 via 10.0.0.2, Tunnel0 created 00:00:02, expire 00:05:57
  Type: dynamic, Flags: router implicit used
  NBMA address: 150.1.2.2
10.0.2.0/24 via 10.0.2.2, Tunnel0 created 00:00:02, expire 00:05:57
  Type: dynamic, Flags: router used
  NBMA address: 150.1.2.2
10.0.3.0/24 via 10.0.3.3, Tunnel0 created 00:00:02, expire 00:05:57
  Type: dynamic, Flags: router authoritative unique local
  NBMA address: 150.1.3.3
    (no-socket) 
The “implicit” flag means that the router learned mapping without explicit request, as a part of other router’s reply or request. The “router” flag means that the mapping is either for the remote router or for a network behind the router. The “(no-socket)” flag means that the local router will not use this entry and trigger IPSec socket creation. The “local” flag means the mapping is for the network directly connected to the local router. The router uses those mappings when it loses connection to the local network, so that the NHC may send a purge request to all other clients, telling that the network has gone and they must remove their mappings.
Here is an example. Ensure R3 has the above-mentioned mappings, and then shut down the Loopback1 interface, observing the debugging command output on R3 and R2. R3 sends purge request directly to R2, since it knows R2 requested that mapping.
Rack1R3#
NHRP: Redist callback: 10.0.3.0/24
NHRP: Invalidating map tables for prefix 10.0.3.0/24 via Tunnel0
NHRP: Checking for delayed event 150.1.3.3/10.0.3.3 on list (Tunnel0).
NHRP: No node found.
NHRP: Attempting to send packet via DEST 10.0.0.2
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.2.2
NHRP: Send Purge Request via Tunnel0  vrf 0, packet size: 73
 src: 10.0.0.3, dst: 10.0.0.2
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "reply required", reqid: 36
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 0
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client protocol: 10.0.3.3
NHRP: 73 bytes out Tunnel0
R2 receives Purge Request from R3. Note that the “reply required” flag is set. Hence, R2 must confirm that it deleted the mapping with a Purge Reply packet. R2 will erase the corresponding mapping learned via “10.0.0.3” and generate a response packet
Rack1R2#
NHRP: Receive Purge Request via Tunnel0  vrf 0, packet size: 73
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "reply required", reqid: 36
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 0
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client protocol: 10.0.3.3
NHRP: netid_in = 123, to_us = 1
NHRP: Attempting to send packet via DEST 10.0.0.3
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.3.3
R2 first tries to send the Purge Reply to R3 directly, using the NBMA address of R3. Note that CIE header mentions the network erased from the local mappings list
Rack1R2#
NHRP: Send Purge Reply via Tunnel0 vrf 0, packet size: 73
 src: 10.0.0.2, dst: 10.0.0.3
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "reply required", reqid: 36
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 0
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client protocol: 10.0.3.3
NHRP: 73 bytes out Tunnel0
NHRP: Invalidating map tables for prefix 10.0.3.0/24 via Tunnel0
NHRP: Attempting to send packet via DEST 10.0.0.1
NHRP: Encapsulation succeeded.  Tunnel IP addr 150.1.1.1
R3 receives the reply to its purge request and now it knows that R2 is consistent.
Rack1R3#
NHRP: Receive Purge Reply via Tunnel0 vrf 0, packet size: 73
 (F) afn: IPv4(1), type: IP(800), hop: 255, ver: 1
     shtl: 4(NSAP), sstl: 0(NSAP)
 (M) flags: "reply required", reqid: 36
     src NBMA: 150.1.3.3
     src protocol: 10.0.0.3, dst protocol: 10.0.0.2
 (C-1) code: no error(0)
       prefix: 0, mtu: 1514, hd_time: 0
       addr_len: 0(NSAP), subaddr_len: 0(NSAP), proto_len: 4, pref: 0
       client protocol: 10.0.3.3
NHRP: netid_in = 0, to_us = 1
Timing out NHRP entries with Process-Switching
The last question is how NHRP times out unused entries in case of process-switching mode. Recall the “used” flag set for NHRP mapping. Every time a packet is process-switched using the respective NHRP entry, it is marked as “used”. The background NHRP process runs every 60 seconds, and check the expire timers for each NHRP entry. If the “used” flag is set and expire timer for the entry is greater than 120 seconds then the process clears the flag (and every new packet will refresh it). If the timer is less than 120 seconds and the flag is set, IOS generates a refreshing NHRP request. However, if the flag is not set, the system allows the entry to expire, unless another packet hits it and makes active.
The above-described behavior of NHRP with process switching allows for one interesting feature. The hub router may now summarize all information sent down to spokes say into one default route. This will not affect the spokes, for they will continue querying next-hop information for every destination prefix sent over the mGRE tunnel interface, and learning the optimal next-hop. It would be great to combine this “summarization” feature with the performance of CEF switching. This is exactly what they implemented with DMVPN Phase 3. However, Phase 3 is subject to a separate discussion.
Integrating IPsec
Haven’t we forgotten something for DMVPN Phase 1/Phase 2? That was IPsec, the components that provides confidentiality and integrity checking to mGRE/NHRP. Now, compared with the complexity of NHRP operations, IPsec integration is straightforward.
First, the hub needs to know how to authentication all the spokes using IKE. The most scalable way is to use X.509 certificates and PKI, but for the simplicity, we will just use the same pre-shared key on all routers. Note that we need to configure the routers with a wild-card pre-shared key, in order to accept IKE negotiation requests from any other dynamic peer.
As for IPsec Phase 2, we need dynamic crypto maps there, since the hub has no idea of the connecting peer IP addresses. Fortunately, Cisco IOS has a cute feature called IPsec profiles, designed for use with tunnel interfaces. The profile attaches to a tunnel interface and automatically considers all traffic going out of the tunnel as triggering the IPsec Phase 2. The IPsec phase proxy identities used by the IPsec profile are the source and destination host IP addresses of the tunnel. It makes sense to use IPSec transport mode with mGRE as the latter already provides tunnel encapsulation. Besides, IOS supports some features, like NAT traversal only with IPSec transport mode.
Let’s review an example below and explain how it works.
mGRE + NHRP Phase 2 + Spoke-to-spoke tunnels + IPsec
R1:
crypto isakmp policy 10
 encryption 3des
 authentication pre-share
 hash md5
 group 2
!
crypto isakmp key 0 CISCO address 0.0.0.0 0.0.0.0
!
crypto ipsec transform-set 3DES_MD5 esp-3des esp-md5-hmac
 mode transport
!
crypto ipsec profile DMVPN
 set transform-set 3DES_MD5
!
interface Tunnel 0
 tunnel protection ipsec profile DMVPN

R2 & R3:

crypto isakmp policy 10
 encryption 3des
 authentication pre-share
 hash md5
 group 2
!
crypto isakmp key 0 CISCO address 0.0.0.0 0.0.0.0
!
crypto ipsec transform-set 3DES_MD5 esp-3des esp-md5-hmac
 mode transport
!
crypto ipsec profile DMVPN
 set transform-set 3DES_MD5
!
interface Tunnel 0
 tunnel protection ipsec profile DMVPN
Start with any spoke, e.g. R3. Since the router uses EIGRP on Tunnel 0 interface, a multicast packet will eventually be send out of the tunnel interface. Thanks to the static NHRP multicast mapping, mGRE will encapsulate the EIGRP packet towards the hub router. The IPsec profile will see GRE traffic going from “150.1.3.3” to “150.1.1.1”. Automatically, ISAKMP negotiation will start with R1, and authentication will use pre-shared keys. Eventually both R1 and R3 will create IPsec SAs for GRE traffic between “150.1.3.3” and “150.1.1.1”. Now R3 may send NHRP resolution request. As soon as R3 tries to send traffic to a network behind R2, it will resolve next-hop “10.0.0.2” to the IP address of 150.1.2.2. This new NHRP entry will trigger ISAKMP negotiation with NBMA address 150.1.2.2 as soon as router tries to use it for packet forwarding. IKE negotiation between R3 and R2 will start and result in formation of new SAs corresponding to IP address pair “150.1.2.2 and 150.1.3.3” and GRE protocol. As soon as the routers complete IPsec Phase 2, packets may flow between R2 and R3 across the shortcut path.
When an unused NHRP entry times out, it will signal the ISAKMP process to terminate the respective IPsec connection. We described the process for timing out NHRP entries before, and as you remember, it depends on the “hold-time” value set by the routers. Additionally, the systems may expire ISAKMP/IPsec connections due to IPsec timeouts.
This is the crypto system status on the hub from the example with NHRP Phase 2 and process-switching:
IPsec Phase 1 has been established with both spokes

Rack1R1#show crypto isakmp sa
dst             src             state          conn-id slot status
150.1.1.1       150.1.2.2       QM_IDLE              1    0 ACTIVE
150.1.1.1       150.1.3.3       QM_IDLE              3    0 ACTIVE
IPsec Phase 2 SA entries for both protected connections to R2 and R3 follows. Note that SAs are for GRE traffic between the loopback.
Rack1R1#show crypto ipsec sa

interface: Tunnel0
    Crypto map tag: Tunnel0-head-0, local addr 150.1.1.1

   protected vrf: (none)
   local  ident (addr/mask/prot/port): (150.1.1.1/255.255.255.255/47/0)
   remote ident (addr/mask/prot/port): (150.1.2.2/255.255.255.255/47/0)
   current_peer 150.1.2.2 port 500
     PERMIT, flags={origin_is_acl,}
    #pkts encaps: 230, #pkts encrypt: 230, #pkts digest: 230
    #pkts decaps: 227, #pkts decrypt: 227, #pkts verify: 227
    #pkts compressed: 0, #pkts decompressed: 0
    #pkts not compressed: 0, #pkts compr. failed: 0
    #pkts not decompressed: 0, #pkts decompress failed: 0
    #send errors 12, #recv errors 0

     local crypto endpt.: 150.1.1.1, remote crypto endpt.: 150.1.2.2
     path mtu 1514, ip mtu 1514, ip mtu idb Loopback0
     current outbound spi: 0x88261BA3(2284198819)

     inbound esp sas:
      spi: 0xE279A1EE(3799622126)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2001, flow_id: SW:1, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4472116/2632)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE
      spi: 0xB4F6A9E5(3036064229)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2003, flow_id: SW:3, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4596176/2630)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE
      spi: 0x1492E4D0(345171152)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2005, flow_id: SW:5, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4525264/2630)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     inbound ah sas:

     inbound pcp sas:

     outbound esp sas:
      spi: 0x81949874(2173999220)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2002, flow_id: SW:2, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4472116/2626)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE
      spi: 0xAA5D21A7(2858230183)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2004, flow_id: SW:4, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4596176/2627)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE
      spi: 0x88261BA3(2284198819)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2006, flow_id: SW:6, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4525265/2627)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     outbound ah sas:

     outbound pcp sas:

   protected vrf: (none)
   local  ident (addr/mask/prot/port): (150.1.1.1/255.255.255.255/47/0)
   remote ident (addr/mask/prot/port): (150.1.3.3/255.255.255.255/47/0)
   current_peer 150.1.3.3 port 500
     PERMIT, flags={origin_is_acl,}
    #pkts encaps: 225, #pkts encrypt: 225, #pkts digest: 225
    #pkts decaps: 226, #pkts decrypt: 226, #pkts verify: 226
    #pkts compressed: 0, #pkts decompressed: 0
    #pkts not compressed: 0, #pkts compr. failed: 0
    #pkts not decompressed: 0, #pkts decompress failed: 0
    #send errors 17, #recv errors 0

     local crypto endpt.: 150.1.1.1, remote crypto endpt.: 150.1.3.3
     path mtu 1514, ip mtu 1514, ip mtu idb Loopback0
     current outbound spi: 0xBEB1D9CE(3199326670)

     inbound esp sas:
      spi: 0x10B44B31(280251185)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2007, flow_id: SW:7, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4436422/2627)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     inbound ah sas:

     inbound pcp sas:

     outbound esp sas:
      spi: 0xBEB1D9CE(3199326670)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2008, flow_id: SW:8, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4436424/2627)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     outbound ah sas:

     outbound pcp sas:
Now let’s see how a spoke router establishes a spoke-to-spoke IPsec tunnel:

No NHRP mapping for spoke’s network first


Rack1R3#sh ip nhrp
10.0.0.1/32 via 10.0.0.1, Tunnel0 created 02:02:42, never expire
  Type: static, Flags: authoritative used
  NBMA address: 150.1.1.1 


ISAKMP negotiated just with R1


Rack1R3#sh crypto isakmp sa
dst             src             state          conn-id slot status
150.1.1.1       150.1.3.3       QM_IDLE              1    0 ACTIVE
Generate traffic to network behind R2. Note that the first ping passes through, since it’s routed across the hub, but the second packet is sent directly to R2 and is missed, since IPsec Phase 2 has not yet been established
Rack1R3#ping 10.0.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.2.2, timeout is 2 seconds:
!.!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 52/121/324 ms


Notice the new NHRP mappings. Note that the tunnel will expire in about 3
 minutes, if no new traffic is going to be generated


Rack1R3#sh ip nhrp 
10.0.0.1/32 via 10.0.0.1, Tunnel0 created 02:05:38, never expire
  Type: static, Flags: authoritative used
  NBMA address: 150.1.1.1
10.0.2.0/24 via 10.0.2.2, Tunnel0 created 00:02:44, expire 00:03:15
  Type: dynamic, Flags: router
  NBMA address: 150.1.2.2
IOS create IPsec Phase 2 SAs for tunnels between R2-R3 and R1-R3. The tunnel between 2 and R3 is dynamic and is used to send only the data traffic.
Rack1R3#show crypto isakmp sa
dst             src             state          conn-id slot status
150.1.1.1       150.1.3.3       QM_IDLE              1    0 ACTIVE
150.1.3.3       150.1.2.2       QM_IDLE              2    0 ACTIVE

Rack1R3#show crypto ipsec sa

interface: Tunnel0
    Crypto map tag: Tunnel0-head-0, local addr 150.1.3.3

   protected vrf: (none)
   local  ident (addr/mask/prot/port): (150.1.3.3/255.255.255.255/47/0)
   remote ident (addr/mask/prot/port): (150.1.1.1/255.255.255.255/47/0)
   current_peer 150.1.1.1 port 500
     PERMIT, flags={origin_is_acl,}
    #pkts encaps: 290, #pkts encrypt: 290, #pkts digest: 290
    #pkts decaps: 284, #pkts decrypt: 284, #pkts verify: 284
    #pkts compressed: 0, #pkts decompressed: 0
    #pkts not compressed: 0, #pkts compr. failed: 0
    #pkts not decompressed: 0, #pkts decompress failed: 0
    #send errors 0, #recv errors 0

     local crypto endpt.: 150.1.3.3, remote crypto endpt.: 150.1.1.1
     path mtu 1514, ip mtu 1514, ip mtu idb Loopback0
     current outbound spi: 0x10B44B31(280251185)

     inbound esp sas:
      spi: 0xBEB1D9CE(3199326670)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2001, flow_id: SW:1, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4526856/2383)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     inbound ah sas:

     inbound pcp sas:

     outbound esp sas:
      spi: 0x10B44B31(280251185)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2002, flow_id: SW:2, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4526853/2381)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     outbound ah sas:

     outbound pcp sas:

   protected vrf: (none)
   local  ident (addr/mask/prot/port): (150.1.3.3/255.255.255.255/47/0)
   remote ident (addr/mask/prot/port): (150.1.2.2/255.255.255.255/47/0)
   current_peer 150.1.2.2 port 500
     PERMIT, flags={origin_is_acl,}
    #pkts encaps: 3, #pkts encrypt: 3, #pkts digest: 3
    #pkts decaps: 4, #pkts decrypt: 4, #pkts verify: 4
    #pkts compressed: 0, #pkts decompressed: 0
    #pkts not compressed: 0, #pkts compr. failed: 0
    #pkts not decompressed: 0, #pkts decompress failed: 0
    #send errors 0, #recv errors 0

     local crypto endpt.: 150.1.3.3, remote crypto endpt.: 150.1.2.2
     path mtu 1514, ip mtu 1514, ip mtu idb Loopback0
     current outbound spi: 0x847D8EEC(2222821100)

     inbound esp sas:
      spi: 0xA6851754(2793740116)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2004, flow_id: SW:4, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4602306/3572)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     inbound ah sas:

     inbound pcp sas:

     outbound esp sas:
      spi: 0x847D8EEC(2222821100)
        transform: esp-3des esp-md5-hmac ,
        in use settings ={Transport, }
        conn id: 2003, flow_id: SW:3, crypto map: Tunnel0-head-0
        sa timing: remaining key lifetime (k/sec): (4602306/3572)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     outbound ah sas:

     outbound pcp sas:
Now you see how all the component of DMVPN work together.

We have not covered some other major topics like NAT traversal with NHRP and DMVPN redundancy with multiple hubs.