Saturday, June 13, 2009

PIM/IGMP Elections

On a shared LAN segment, amongst the PIM enabled routers, a selected router must assume the responsibilty for i) sending any PIM register/prune messages to the RP and for ii) sending IGMP query messages.

I was until recently under the misunderstanding that the PIM DR router performed both of these functions - wrong!! These functions are completely decoupled and in fact they have a different election process and selection criteria.

First the Querier Election Process.
At start up each router sends a query message to the all systems group from its own interface address. The router with the lowest ip address is elected IGMP querier.

Second the PIM DR Election Process
The router with the highest ip address is elected as PIM DR. This selection process can also be influenced by configuring a pim DR priority. By default all routers have priority 1, hence highest ip address wins by default. However if DR priority is used then highest DR priority wins.

The show igmp interface command can be used to show elected DR and querier. Here is elected querier (lowest ip address on LAN segment) and is elected DR (highest ip address on LAN segment).

Router_1(config)#do s ip igmp int fa0/0
FastEthernet0/0 is up, line protocol is up
Internet address is
IGMP is enabled on interface
Current IGMP host version is 2
Current IGMP router version is 2
IGMP query interval is 20 seconds
IGMP querier timeout is 40 seconds
IGMP max query response time is 4 seconds
Last member query count is 2
Last member query response interval is 1000 ms
Inbound IGMP access group is not set
IGMP activity: 0 joins, 0 leaves
Multicast routing is enabled on interface
Multicast TTL threshold is 0
Multicast designated router (DR) is
IGMP querying router is (this system)
No multicast groups joined by this system