Configuring IGMP on 2950 Switches

[jkout]

I have a network consisting entirely of 2950 switches although some are 2950Gs and the rest are just 2950s.

I am running CCTV video encoders over the network which generates a lot of multicast traffic. The multicast traffic appears to be forwarding out of every active port on every switch (the default config) whether there is a multicast client attached or not. This results in the switches and the wireless backbone links being saturated with traffic that doesn't need to cross the link.

Is there a way using IGMP snooping (2950s don't appear to support CGMP) to prevent multicast traffic from crossing links it doesn't need to?

I've seen the 2950 IGMP config doc but it doesn't seem to help. Also, this doc mentions a "multicast router." Would that be the 1760 router I have onsite for Internet access, or is that merely a term for some other device on the network?

I have somewhere around 40 video encoders that produce a minimum of 64 multicast streams. Normal multicast load is probably around 100 - 128 streams at anywhere from 300kb - 1000kb per stream. I've seen the 5 minute input/output rate on switch ports at range from 20mbps @ 2500 packets per sec to 60 mbps @ 6000 packets per sec.

Thanks.

[vladrac-ccna]

Hello there,

as I remember (back in the days I was studying for the BCMSN exam), I believe 2950 not only does IGMP Snooping but it does in hardware.

for configuration check:

http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_guide_chapter09186a00801cde69.html#1019998

Hope it helps,

Vlad

[andrew.butterworth · Accepted solution]

If you don't have any multicast routers in the network then the 2950's will simply flood the multicast traffic out of all the ports in the VLAN it was received in.

2950's support IGMP snooping and it is on by default but without a multicast router in the mix it doesn't achieve anything.

How big is your network? It sounds as though there is no structure and you have simply cascaded 2950's creating a big, flat layer-2 network. This is bad not only from a multicast perspective but from an STP and scalability perspective as well.

HTH

Andy

[jkout]

Andrew,

My network isn't that large. I have a 1760 router connecting to 7 2950 switches in the following config:

| - 2950G

| - 2950G

1760 - 2950 -| - 2950G

| - WB - 2950

| - WB - 2950 - WB - 2950

The WB is a non-Cisco wireless bridge. These are the links that are being saturated by the multicast traffic. They have source CCTV multicast traffic going back to the main 2950G. The WBs also get flooded by CCTV multicast traffic from the other sources that doesn't need to be on them. I need to block the multicast traffic from the rest of the network to the WBs but still allow the multicast traffic that needs to be forwarded to the CCTV software.

I am also unclear on the definition of a multicast router. Is this an actual router (1700, 2600, 3600) or can this be another device, like a switch?

Thanks,

Jeff

[andrew.butterworth]

A multicast router is simply a router that forwards IP multicast traffic between networks (in the same way a router routes unicast traffic). Multicast is different in the fact that the destination address doesn't physically exist and hosts ask to receive the traffic by 'joining' the multicast group.

All your IGMP snoopoing switches need to see a multicast router, if this is enabled on the 1760 (by enabling the IGMP Querier and Mrouter on the interface) then all Multicast traffic will be forwarded to it. This may result in all your interlinks being flooded with multicast, which may be better than the current situation of all ports in the vlan being flooded with multicast.

You can enable multicast routing on the 1760 by:

!global config

ip multicast-routing

!

interface FastEthernet0/1

ip pim sparse-dense

!

I suggest you look at segmenting your network down - maybe put a 3560 (Enhanced Image) in the middle and break the IP network into smaller networks and let the 3560 forward the multicast.

HTH

Andy

[jkout]

Enabling multicast on the Cisco 1760 did the trick.

Thanks,

Jeff