Help with IGMP

Unanswered Question
Oct 28th, 2008

I'm facing some issues with IGMP, but unfortunately know next to nothing about it. The extent of my knowledge is that adding and removing various iterations of 'ip igmp snooping' globally or per VLAN sometimes results in a working config :-)

We have several 2960s that feed small streaming networks, with one or more VOD or other streaming video server. Those servers have various test clips, video sent by customers for troubleshooting, etc. We just got in a big satellite dish that has feeds for several channels, and I'm trying to reliably get the video from it.

Behind the receivers for the dish is a 48 port 2960. Someone added a crossconnect from that switch to one of the streaming networks. It looked like traffic from the streaming network was flooding onto the satellite distribution switch... Wireshark showed a lot of stuff with source addresses that I recognized as being on that streaming network. I thought that, with IGMP, you had to specifically "join" a "channel", so I'm not sure why this happened. All of that traffic seemed to bring the satellite switch to it's knees... no hosts on it could ping each other, and we could not play any video signals with VLC on a laptop connected directly to that switch.

Once I removed that crossconnect, things improved. One laptop with VLC was able to play Animal Planet HD, but it was extremely choppy... horrible pixelation, short "freezes", etc.

I connected another laptop, and if I start to play Animal Planet HD, I can see 8.9 Mb/s of traffic coming in, but it just sits on one frame. I cannot play any other channels. After a few minutes, VLC bombs out and wants to send it's useless error report off into the ether.

I realize I'm not giving a lot of useful info here, but I have no idea what else could be pertinent. I can find my way around IOS, but I know nothing about multicast. I could use any suggestions on how to correctly set up a Cisco switch for multicast, or any pointers to material that might help.

I have this problem too.
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Jon Marshall Tue, 10/28/2008 - 09:10


Firstly we need to know are you trying to use multicast only within single vlan or are you trying to use multicast across multiple vlans.

If you are trying to use across multiple vlans what multicast routing protocol are you using - pim sparse or pim dense.


Jon Marshall Tue, 10/28/2008 - 09:17

Okay, if you have IGMP snooping configured have you configured the IGMP snooping querier function on your switches. You need this because IGMP snooping listens for responses to IGMP queries. Think of it like this - if nothing asks the question then there will be no answers.

The IGMP queries are usually made by a L3 interface and IGMP querying is enabled as soon as you configure pim under a L3 interface.

However if you aren't configuring PIM some switches have the ability to run an IGMP querier and your 2960 switches do have this ability.


jnojr Tue, 10/28/2008 - 12:07

I have:

ip igmp snooping tcn query solicit

no ip igmp snooping

Those are the only lines pertinent to IGMP on this particular switch.

Jon Marshall Tue, 10/28/2008 - 13:18

So are you saying that in the link i sent you cannot configure

2960(config)# igmp snooping querier

if you don't have this option what IOS are you running on your switch ?

If you don't have the querier function you have 2 choices

1) statically map multicast addresses to ports - not at all scalable and not recommended.

2) Enable pim on the L3 interface for the relevant vlan.


jnojr Tue, 10/28/2008 - 13:22

I actually have Version 12.2(37)SE1

SES-Distribution(config)#ip igmp snooping querier

Command did not take effect due to reason: snooping is globally disabled.

SES-Distribution(config)#ip igmp snooping

SES-Distribution(config)#ip igmp snooping querier

IGMP Querier function cannot be operationally enabled on some VLANs because the required conditions have not been met.

Jon Marshall Tue, 10/28/2008 - 13:25

The vlan you are using. Does it have a layer 3 SVI for it. You need to configure this in your command. Please read the link i sent which explains all the requirements and how to get it working.


jnojr Tue, 10/28/2008 - 13:36

Yes, that VLAN has an IP.

SES-Distribution#sh ip igmp snooping querier

Vlan IP Address IGMP Version Port


2 v2 Switch


Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:


SES-Distribution#sh ip igmp snooping groups

Vlan Group Type Version Port List


2 igmp Gi0/34, Gi0/38

2 igmp Gi0/12

SES-Distribution#sh ip igmp snooping groups

Vlan Group Type Version Port List


2 igmp Gi0/34, Gi0/38

2 igmp v2 Gi0/12

2 igmp Gi0/12 is one of the streams. I can direct VLC to it but I do not get any video. I do see the 8.9Mb/s of traffic entering the laptop I'm trying to play the video on, though.


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