IGMP snooping on 6509

Unanswered Question
Nov 20th, 2008

When igmp snooping is applied the interface that sends multicast packets out the job fails after a few minutes. If IGMP is turned off the job passes but multicast hammers the entire network so i need it working. Any suggestions.

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netsquant Thu, 11/20/2008 - 11:06

yes, i was looking into that but not sure if that is the missing piece.

Giuseppe Larosa Thu, 11/20/2008 - 11:14

Hello Shane,

IGMP snooping works by sniffing IGMP reports from receivers that are sent if there is a multicast router sending IGMP queries every 60 seconds.

If no multicast router is on the Vlan you can activate a feature called IGMP Snooping querier on the C6500 itself.



Hope to help


netsquant Thu, 11/20/2008 - 11:30

what about cgmp, could this be the missing link. IGMP querier still fails.

Jon Marshall Thu, 11/20/2008 - 11:33

CGMP requires a L3 interface to inform the switch to map ports to multicast addresses.

Are you running PIM on a L3 interface at all or is this just multicast within a L2 vlan only.

What IOS are you running ?


netsquant Thu, 11/20/2008 - 11:39

My 6509 is running as a l2 switch

(s72033_rp-IPSERVICESK9-VM), Version 12.2(18)SXF10

Jon Marshall Thu, 11/20/2008 - 11:51

When you have IGMP snooping enabled on your 6500 are you saying it works in that it only sends the multicast stream to those ports that are interested in it. I appreciate that it stops working but the key part is does it actually work for those first 3 minutes or so ?

If the switch is L2 only where is the L3 vlan interface for the L2 vlan on the 6500 ?


netsquant Thu, 11/20/2008 - 11:52

the job starts and then crashes and i have 2 7206's on the inside also.

Jon Marshall Thu, 11/20/2008 - 11:55

Okay the job starts then crashes but in your original thread you said if you turn off IGMP snooping then the network gets hammered. So can i assume from this that before the job crashes the network is not getting hammered.

Where is the L3 interface for the L2 vlan on the 6500 ?


netsquant Thu, 11/20/2008 - 11:57

when igmp is on the multicast job only goes where needed and crashes after a few minutes, when off the job completes but my entire network gets hammered with multicast. the 6509 hits my 7206 then out to the WAN. the 7206 is l3.

Jon Marshall Thu, 11/20/2008 - 12:03

Okay so before the job crashes and you have IGMP snooping turned on, what is doing the IGMP querying ?. As noted by Giuseppe you either have "IGMP snooping querier" enabled or you have a PIM enabled interface.

You could try enabling PIM only on the L3 interface for that vlan on the 7206.

I have checked the bug tool for IGMP bugs related to your IOS but nothing matches what you are seeing.


netsquant Thu, 11/20/2008 - 12:11

ip pim-sm is on the lan interface of the 7206 does it need to be on teh wan side also?

Jon Marshall Thu, 11/20/2008 - 12:12

No, only on the LAN side if you are simply trying to contain the multicast within your L2 vlan.


Jon Marshall Thu, 11/20/2008 - 12:16

Okay let us know how you get on.

One other thing. You don't have any traffic storm control configured on your 6500 switchports do you ?


Jon Marshall Thu, 11/20/2008 - 12:24

So when the job crashes does the interface stay up and passing other traffic. Do you get any logs from the job as to why it failed.

You could try upgrading your software but i'm not convinced that will make any difference.


Jon Marshall Thu, 11/20/2008 - 12:29

The only other thing that springs to mind is that you could statically map the multicast address to the relevant ports on the switch. If the intended destinations are always the same this might be a bit of work at the start.

But obviously if the membership is dynamic this is no good to you.


netsquant Thu, 11/20/2008 - 12:35

the server guys who are running the multicast jobs think it may have something to do with the vm enviroment.

I will keep you posted.

Thank you.

CriscoSystems Thu, 11/20/2008 - 11:40

Shane, IGMP and CGMP are exclusive of each other. If you've got one configured then you can't run the other. So if IGMP is what you really want to use, CGMP can't be the "missing piece."

An IGMP switch has the ability to read the L3 packet inside the frame and to map which port it's using to get to the mcast group member. A switch that can't (or won't) do this can use CGMP. In CGMP, the switch relies on a PIM-enabled router upstream from the multicast members. The router informs the switch of the unicast MAC addresses of all the mcast group members it knows about, the switch then maps those MAC addresses (and corresponding exit ports) to the relevant multicast address(es).

I don't know enough about your setup to know why IGMP is failing, but if you've got CGMP-compatible equipment, it's worth a shot to shut down IGMP and see what luck you have with CGMP.

Hope to help, and please remember to rate helpful posts.

-- stuey


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