multicast across VTP/ISL trunked switches

Unanswered Question
Oct 14th, 2008

if a multicast originates on VLAN X on a "server" trunked switch and must propogate to the "client" switch across an ISL trunk to VALN X, does the mulicast on the destination switch flood all switch ports in VLAN X.

I have this problem too.
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Jon Marshall Tue, 10/14/2008 - 10:08

It does unless you have configured IGMP snooping on your switch

AND either

1) you run pim on the L3 vlan interface

2) if not 1 then you need to run the IGMP snooping querier function on the switch.

Note not all switches support the querier function.


plavine Tue, 10/14/2008 - 10:23

I do have pim on both SVI's that are HSRP'd between the 2 switches. It does not seem to make a difference though.

plavine Tue, 10/14/2008 - 10:26

Confirmed. It is on be default. I have not turned it off.

Jon Marshall Tue, 10/14/2008 - 10:32

on L3 switch can you post output of

1) sh ip igmp groups

2) sh ip igmp membership

3) Can you confirm what the group address is

4) What exactly do you mean by "server" trunked ?

Lastly How are you confirming that all ports are receiving the multicast ?


plavine Tue, 10/14/2008 - 11:46

I can see that when I start sending multicast traffic all switch ports on the client switch receive traffic in VLAN 200 and on the server switch (where the sender is) only the switch ports with listerners receive the traffic as they should with PIM enabled on the SVI. I also notice a significant amount of packet loss on the client switch if there is any fragmentation needed.


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