IGMP snooping

[tedauction]

Hello, we have a remote site with a L2 multicast WAN circuit connection to a Telco.

We have been told to disable 'igmp snooping' on our router's L2 uplink interface to this multicast WAN and also to enter 'no switchport multicast'.

I am wondering why this would be necessary. They say it has something to do with them providing the IGMP querier from within the Telco's WAN cloud.

Thank you for any explanation.

[paul driver · Accepted solution]

Hello

My understanding switchport multicast is to negate for L2 multicast flooding and igmp snooping is for L3 IP multicast flooding.

So with the telco providing the igmp querier, They basically they are going to initiate the membership queries to the hosts willing to receive MC traffic and wish you not to negate flooding of any MC traffic

res

Paul

[tedauction]

Hello, thank you.

So just to confirm, if I have IGMP snooping and multicast activated on my site switch, then this will block their Telco IGMP querier messages ?

Is that the reason why you do not have daisy-chained switches with igmp snooping activated ?

So if that is the case, then how does the upstream Telco querier know about which ports on my local switch require multicast ?

[paul driver · Accepted solution]

Hello

My understanding is snooping listens for and notes the switchport the igmp report comes in on and  as such when mc traffic is sent to a switch that is directly connect to the mc source that switch is now able to send mc traffic to the correct port without flooding it to every port on that switch.

Now when there a switch behind the primary switch and a host on this switch2 sends an igmp report that port is noted on the secondary switch only and not on the primary switchport interconnecting the two switches .Thus the the primary switch isn't aware of any igmp report being received and so does t have anything in its snooping table to relate to.

Disabling snooping will allow flooding of the igmp queries to all ports in both switches and as such any interested host will be allowed to acknowledge its intent of receiving mc traffic and be able to connect to the mc group

res

paul