Multicast Traffic Copied to Other Switch

[emuman100]

I have two switches connected to each other and "ip igmp snooping querier" in both their configurations. One switch runs IOS 15 and the other runs IOS 12. That is the only multicast related configuration in the switches.

I have a multicast source connected to one switch (with IOS 15), transmitting traffic to multicast group 239.x.x.x. When the multicast source is transmitting to that group on one switch, it is copied to the other switch (with IOS12) even though there are no devices connected to that switch that are subscribed to that multicast group to copy traffic. This congests the link between the switches unnecessarily.

I remember when I used an older switch with IOS 12 on it and multicast sources connected to it, I did not have this problem, but now that I replaced it with a newer switch with IOS 15, it has this problem. This tells me more configuration might be needed on the switch with IOS 15.

Does anyone know what the issue is?

Thanks.

[mlund]

Hi

I would try to remove "ip igmp snooping querier" in the second switch, and just have it configured in the switch where the sender is. It might be that when the second switch send its querier the first switch will add the port between switches to forwarding list.

/Mikael 

[emuman100]

I tried that but it did not make any difference. Traffic was still copied to that switch like before.

[mlund]

Maybe you can share the groups that have this problem. The layer2 part of the mcast frame is  copied from the last 23 bits of the L3 address. That means that if you have 239.0.0.x or 239.128.0.x you have overlapping L2 addresses with 224.0.0.x. And those will always be forwarded. 

[emuman100]

That might be the problem. The multicast group that I'm using is 239.156.0.90. Does this have an overlapping address with 224.0.0.x?

[mlund]

No, that address will not overlap. Can you share the output from "show ip igmp snooping group 239.156.0.90"

[emuman100]

Sorry for missing this!

On the switch LAN3 with the multicast source connected to it, "show ip igmp snooping group" does not show anything at all.

On switch LAN2 where the traffic is copied to, "show ip igmp snooping group" shows one group shows one group that is associated with a VoIP phone I have:

LAN2#show ip igmp snooping group
Vlan      Group                    Type        Version     Port List
-----------------------------------------------------------------------
1         224.168.168.168          igmp        v2          Gi0/2

Could the fact that LAN3 with the multicast source connected to it not showing anything be the issue?

[mlund]

Strange, if there is no output at all, I think the switch will flood all ports with the mcast. Are the igmp snooping enabled on the LAN3 switch? Can you do a "sh ip igmp snooping mrouter".  

[emuman100]

Here is the output:

sh ip igmp snooping mrouter
Vlan    ports
----    -----
   1    Gi1/0/3(dynamic)

Gi1/0/3 is the port that is connected to my other switch, LAN2. Both switches are configured as VLAN 1 and all ports are access ports. There are no other VLANs on this LAN. Here is more information:

sh ip igmp snooping querier
Vlan      IP Address               IGMP Version   Port
-------------------------------------------------------------
1         192.168.0.2              v2            Gi1/0/3

sh ip igmp snooping querier vlan 1
IP address               : 192.168.0.2
IGMP version             : v2
Port                     : Gi1/0/3
Max response time        : 10s

Thanks.

[mlund · Accepted solution]

That is the reason why mcast traffic is sent to the other switch. Mcast traffic is always sent to the mcast router (the device that sent the igmp queries). You have to remove the igmp querier on the device that have ip 192.168.0.2, and only have it enabled on LAN3.

B.T.W. if You have a router with ip pim sparse or sparse/dense mode configured, that will also generate igmp querier.

[emuman100]

Removing "ip igmp snooping querier" from LAN2 did the trick! When a subscriber connected to LAN2 is subscribed to the multicast group, traffic is copied to LAN2 like it should and only to that port requesting it. When the subscriber unsubscribes, traffic stops like it should. Also, there will never be a multicast source on LAN2, so "ip igmp snooping querier" was not even needed. Thank you for solving this problem!