IGMP Timers

Today i'm explaining the "hated" thousands of IGMP Timers :)

First, we should all remember, that in a LAN environnement, the Elected IGMP Querier is the lowest IP address.  (remember that this is reserved for IGMP2, IGMP1 has a PIM election procedure)

1)   (IF)  ip  igmp  query-interval <sec>

Time interval between each Sending the General Query Message by Router to check which Channels are seen by hosts

default = 60s

2)   (IF)  ip  igmp  query-max-response-time  <1/10th of sec>

this is MRT, this timer is sent within the General Query message. in fact, each host, when receiving the General Query, he set a   0.1<random timer<MRT  before he answer with a report the Router.

Note that, if a host already hear a Report for his Group from some other host, before that host reach his chosen random timer.  so that host will not send a Report to Router.

3)   (IF)  ip  igmp  last-member-query-interval  <sec>

After a Router receive a Leave Message for some Group, he wait for that amount of time before he send a Specific Query about that Group to see of there is some other hosts still using that Group.

default = 1s

4)   (IF)  ip  igmp  last-member-query-count <numb>

Note that, as we have seen in 3), when Router receive a Leave , he send a Specific Query for that Group, in fact, by default he do this twice in order to be sure, we can adjust how many time he send that Specif Query by this command.

default = 2 times

5)   (IF)  ip  igmp  querier-amount  <sec>

Backup Querier becomes an active one if it does not hear queries from the other router within this amount of time

default =  2 x Querier Interval   

Some final notes:

--> Note that a Router keep an IGMP ( * , G)  state before suppress by default to 3 x Query Interval