Understanding IPv6 Link local address

IPv6 Link local address 

The purpose of this document is to provide an understanding of IPv6 Link-local address in a network. 

A link-local address is an IPv6 unicast address that can be automatically configured on any interface using the link-local prefix FE80::/10 (1111 1110 10) and the interface identifier in the modified EUI-64 format. Link-local addresses are not necessarily bound to the MAC address (configured in a EUI-64 format). Link-local addresses can also be manually configured in the FE80::/10 format using the ipv6 address link-local command.

These addresses refer only to a particular physical link and are used for addressing on a single link for purposes such as automatic address configuration and neighbor discovery protocol. Link-local addresses can be used to reach the neighboring nodes attached to the same link. The nodes do not need a globally unique address to communicate. Routers will not forward datagram using link-local addresses. IPv6 routers must not forward packets that have link-local source or destination addresses to other links. All IPv6 enabled interfaces have a link-local unicast address.

So, In a computer network, a link-local address is a network address that is valid only for communications within the network segment (link) or the broadcast domain that the host is connected to.

Link-local addresses are usually not guaranteed to be unique beyond a single network segment. Routers therefore do not forward packets with link-local addresses.

For protocols that have only link-local addresses, such as Ethernet, hardware addresses that the manufacturer delivers in network circuits are unique, consisting of a vendor identification and a serial identifier.

Link-local addresses for IPv4 are defined in the address block 169.254.0.0/16, in CIDR notation. In IPv6, they are assigned with the FE80::/64 prefix. ^[1]

Video https://supportforums.cisco.com/video/11930026/understanding-ipv6-link-local-address 

In this example, the routers R1, R2 and R3 are connected via serial interface and have the IPv6 addresses configured as mentioned in the network diagram. Loopback addresses are configured on the routers R1 and R3, and the routers use OSPFv3 to communicate with each other. This example uses the ping command to demonstrate the connectivity between the routers using link-local addresses. The routers R1 and R3 can ping each other with the IPv6 global unicast address, but not with their link-local address. However, router R2 being directly connected to R1 and R3 can communicate with both the routers using their link-local address, because link-local addresses are used only within that local network specific to the physical interface.

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Network Diagram

Router R1

! hostname R1 ! ipv6 cef ! ipv6 unicast-routing ! interface Loopback10  no ip address  ipv6 address 2010::/64 eui-64 !--- Assigned a IPv6 unicast address in EUI-64 format.   ipv6 ospf 1 area 1 !--- Enables OSPFv3 on the interface and associates  the interface looback10 to area 1. ! interface Loopback20  no ip address  ipv6 address 2020::/64 eui-64  ipv6 ospf 1 area 2 !--- Associates the Interface loopback20 to area 2. ! interface Serial0/0  no ip address  ipv6 address 2001::1/124  ipv6 ospf 1 area 0 !--- Associates the Interface serial0/0 to area 0.  clock rate 2000000 ! ipv6 router ospf 1  router-id 1.1.1.1 !--- Router R1 uses 1.1.1.1 as router id.  log-adjacency-changes ! end

Router R2

Router R3

hostname R2 ! ipv6 cef ! ! ! ! ipv6 unicast-routing ! ! ! interface Serial0/0  no ip address  ipv6 address 2001::2/124  ipv6 ospf 1 area 0  clock rate 2000000 ! ! interface Serial0/1  no ip address  ipv6 address 2002::1/124  ipv6 ospf 1 area 0  clock rate 2000000 ! ! ! ipv6 router ospf 1 router-id 2.2.2.2 log-adjacency-changes ! end

! hostname R3 ! ipv6 cef ! ipv6 unicast-routing ! interface Loopback10  no ip address  ipv6 address 1010::/64 eui-64  ipv6 ospf 1 area 1 ! interface Loopback20  no ip address  ipv6 address 2020::/64 eui-64  ipv6 ospf 1 area 2 ! interface Serial0/0  no ip address  ipv6 address FE80::AB8 link-local  ipv6 address 2002::2/124  ipv6 ospf 1 area 0  clock rate 2000000 ! ipv6 router ospf 1  router-id 3.3.3.3  log-adjacency-changes ! end

Verifying OSPF Configuration

In order to verify the OSPF has been configured properly, use the show ipv6 route ospf command in routers R1 and R3.

show ipv6 route ospf

Router R1 R1#show ipv6 route ospf IPv6 Routing Table - 10 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route, M - MIPv6        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2        D - EIGRP, EX - EIGRP external OI  1010::C002:1DFF:FEE0:0/128 [110/128]      via FE80::C001:1DFF:FEE0:0, Serial0/0 O   2002::/124 [110/128]      via FE80::C001:1DFF:FEE0:0, Serial0/0 OI  2020::C002:1DFF:FEE0:0/128 [110/128]      via FE80::C001:1DFF:FEE0:0, Serial0/0 Router R3 R3#show ipv6 route ospf IPv6 Routing Table - 10 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route, M - MIPv6        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2        D - EIGRP, EX - EIGRP external O   2001::/124 [110/128]      via FE80::C001:1DFF:FEE0:0, Serial0/0 OI  2010::C000:1DFF:FEE0:0/128 [110/128]      via FE80::C001:1DFF:FEE0:0, Serial0/0 OI  2020::C000:1DFF:FEE0:0/128 [110/128]      via FE80::C001:1DFF:FEE0:0, Serial0/0

Verifying Link-Local Address Reachability

The routers can ping each other with the global unicast address. However, when using link-local address only the directly connected networks can communicate. For example, R1 can ping R3 using global unicast address but the two routers cannot communicate using link-local addresses. This is shown using the ping and debug ipv6 icmp commands in router R1 and R3. This section provides scenarios to develop a better understanding about link-local addresses.

Pinging Link-Local Address from Remote Network

When the router R1 tries to communicate with router R3 using the link local address, the router R1 returns with an ICMP time-out message indicating that the link-local address is locally specific and cannot communicate to link-local addresses that are outside the directly connected network.

Pinging R3's Link-Local Address from router R1

In Router R1 R1#ping FE80::AB8   !--- Pinging Link-Local Address of router R3.   Output Interface: serial0/0   !--- To ping LLA, output interface must be entered.   Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to FE80::AB8, timeout is 2 seconds: Packet sent with a source address of FE80::C000:1DFF:FEE0:0 ..... Success rate is 0 percent (0/5)   !--- The ping is unsuccessful and the ICMP packet cannot reach the destination through serial0/0. This timeout indicates that R1 has not received any replies from the router R3.

Pinging Link-Local Address From Directly Connected Network

For router R2, the routers R1 and R3 are directly connected and can ping the link-local address of both router R1 and R2 by mentioning the corresponding interface that is connected to the router. The output is shown here:

Pinging R1 Link-Local Addresses from router R2

In Router R2 R2#ping FE80::C000:1DFF:FEE0:0 !--- Pinging Link-Local Address of router R1. Output Interface: serial0/0 !--- Note that, to ping LLA, output interface should be mentioned In our case, R2 connects to R1 via serial0/0. Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to FE80::C000:1DFF:FEE0:0, timeout is 2 seconds: Packet sent with a source address of FE80::C001:1DFF:FEE0:0 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 0/19/56 ms Debug output from R1 R1# *Mar  1 03:59:53.367: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.371: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.423: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.427: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.463: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.463: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.467: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.467: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 R1# *Mar  1 03:59:53.471: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 03:59:53.471: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 !--- The debug output shows that the router R2 can ping router R1's link-local address.

Pinging R3 Link-Local Addresses from router R2

In Router R2 R2#pingFE80::AB8 !--- Pinging Link-Local Address of router R3. Output Interface: serial0/1 !--- Note that,to ping LLA,output interface should be mentioned. In our case, R2 connects to R3 throught serial0/1. Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to FE80::AB8, timeout is 2 seconds: Packet sent with a source address of FE80::C001:1DFF:FEE0:0 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 0/18/60 ms Debug output from R3 R3# *Mar  1 04:12:11.518: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.522: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.594: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.598: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.618: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.618: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.622: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.622: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 R3# *Mar  1 04:12:11.626: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 04:12:11.630: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 !--- The debug output shows that the router R2 can ping router R3's link-local address.

The link-local address as the name implies, is specific only to that local network. In other words, the routers can have the same link-local address and still the directly connected network can communicate with each other without any conflict. This will not be the same in case of global unicast address. The global unicast address being routable should be unique in a network. The show ipv6 interface brief command shows the information about link-local address on the interface.

show ipv6 interface brief

In router R1 R1#show ipv6 interface brief Serial0/0                  [up/up]     FE80::AB8     2001::1 Loopback10                 [up/up]     FE80::C000:1DFF:FEE0:0     2010::C000:1DFF:FEE0:0 Loopback20                 [up/up]     FE80::C000:1DFF:FEE0:0     2020::C000:1DFF:FEE0:0 In router R3 R3#show ipv6 interface brief Serial0/0                  [up/up]     FE80::AB8     2002::2 Loopback10                 [up/up]     FE80::C002:1DFF:FEE0:0     1010::C002:1DFF:FEE0:0 Loopback20                 [up/up]     FE80::C002:1DFF:FEE0:0     2020::C002:1DFF:FEE0:0 !--- Shows that R1 and R3's serial interface has same link-local address FE80::AB8.

In this example, R1 and R3 are assigned with the same link-local address and R2 can still reach both the routers by specifying the corresponding output interface.

Pinging R1 and R3's Link-local address from R2

Pinging R1's link-local address from R2 R2#ping FE80::AB8 Output Interface: serial0/0 !--- R2 is connected to R1 through serial0/0. Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to FE80::AB8, timeout is 2 seconds: Packet sent with a source address of FE80::C001:1DFF:FEE0:0 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 0/26/92 ms Debug output from R1 R1# *Mar  1 19:51:31.855: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.859: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.915: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.919: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.947: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.947: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.955: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.955: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 R1# *Mar  1 19:51:31.955: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:51:31.955: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 Pinging R3's link-local address from R2 R2#ping FE80::AB8 Output Interface: serial0/1 !--- R2 is connected to R1 through serial0/1. Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to FE80::AB8, timeout is 2 seconds: Packet sent with a source address of FE80::C001:1DFF:FEE0:0 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/28/76 ms Debug output from R3 R3# *Mar  1 19:53:38.815: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.819: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.911: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.915: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.923: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.927: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.955: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.955: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0 R3# *Mar  1 19:53:38.963: ICMPv6: Received echo request from FE80::C001:1DFF:FEE0:0 *Mar  1 19:53:38.963: ICMPv6: Sending echo reply to FE80::C001:1DFF:FEE0:0

Note: The R2 can ping the link-local address of R1 and R3 only because they are directly connected. R2 cannot ping the link-local address of the loopback interfaces in routers R1 and R3 as they are not directly connected. Ping works on link-local addresses only in case of directly connected networks.

Note: Traceroutes do not work in case of link-local addresses and return with the % No valid source address for destination. error message. This is because IPv6 routers must not forward packets that have link-local source or destination addresses to other links.