PIM Query-Interval Tuning
The 'ip pim query-interval' controls the interval that a PIM hello packet is transmitted out each pim enabled interface.
The PIM hello packets are used to discover PIM neighbors and to
determine the Designated Router (DR) on each network segment. The
default interval for the PIM hello packets to be sent is 30 seconds. A
PIM neighbor is considered down after 3 consecutive missed messages.
Therefore, it could take 90 seconds for the DR to failover. If you lower
the query interval to 1 second, then the DR failover time is reduced to
3 seconds.
The goal is not to set the query-interval too low so that there is
unnecessary flapping. Cisco generally recommends a 1 second
query-interval, which would give you a 3 second failover at the receiver
edge. Some customers may choose to use the sub-second option. Cisco
does not recommend an interval less than 500 ms. Due to queue lengths
and processing delays on the switch platforms, lower intervals have been
known to cause problems.
Keep in mind that a router with 30 LAN segments and a query-interval
of 1 will need to send out 30 PIM hellos every second. If you turn down
the query-interval to 500 ms then there will be 60 messages per second.
In the core of the network there are typically point-to-point links
and not any directly connected receivers. When a link goes down on a P2P
link, it is a triggered event and the PIM neighbor is immediately
removed. After unicast routing reconverges, PIM join messages will be
sent on the alternative path for the active multicast streams.
Therefore, there is no need to turn down the query-interval in the core
and it is a waste of CPU cycles and bandwidth.
In summary:
• Turn down the pim query-interval on the receiver edge to reduce DR failover time
• This only needs to be done when there are redundant edge routers and receivers
• A general recommendation is a query interval of 1 second and no less
than 500ms. This should be used with care as the number of interfaces
increase.
