igmp snooping

[craig.eyre]

    Hey All,

Scenario:

We have a switch stack of 3750's and the desktop people are trying to ghost/image. The stack is an access layer stack with no layer 3 and igmp snooping is enabled by default. All machines in this scenario are on 1 vlan and we'll call it X. They start the ghost session using multicast and then join (we'll say 5 computer, could be 20+). The desktops connect fine and the ghosting starts. During the course of the ghosting, there is some hesitation and pausing but after a prolonged amount of time they finish successfully. There is no mrouter defined on upstream layer 3 interface as its all on the same vlan.

Results and findings:

While the ghost session is running I notice that alot of other ports with machines on the same vlan (X) are receiving the ghost/image even though they are not participating. This is bad for us as we have 100+ workstations on that stack/vlan that are receiving an image that they don't need.

If I enable ip pim sparse-dense mode on upstream layer 3 interface, everything seems good and just the clients in the session get the image. So my question is:

Do you HAVE to have a querier/mrouter (either layer 3 interface or switch itself) in order for igmp snooping to actually work even if all traffic is on 1 switch/stack?

Thanks,

Craig              

[Rolf Fischer]

Hi Craig,

the document Multicast Does Not Work in the Same VLAN in Catalyst Switches describes the need for a mrouter port for IGMP snooping to work across switches. As I understand it, it should work without additional configuration when the source is on the same switch. And I wonder about the flooding. Perhaps the stacks have a different logic, I would have to do some readings about that later.

Regards

Rolf

[Peter Paluch]

Craig, Rolf,

As I understand it, it should work without additional configuration when the source is on the same switch

I am not sure about this, Rolf. A single switch initially learns about the receivers thanks to their IGMP Membership Report messages sent when they first subscribed to the group. However, after that, no more Report messages are sent without the presence of an IGMP Querier on the network, meaning that IGMP Snooping-created entries eventually expire and the IGMP Snooping database remains empty. A good question is what happens now if a switch running IGMP Snooping has no mrouter port identified, has an empty IGMP Snooping database, and receives a multicast stream. Does it block it completely? Or does it flood the stream indiscriminately? I am not sure about this, and I would need to test this in a lab.

Craig, just to make sure - do you happen to know the exact multicast IP address used by the Ghost?

Best regards,

Peter

[Richard Primm]

Hi Craig, have you had a chance to review this doc? We use it a lot for reference when troubleshooting scenarios such as yours.

Multicast Does Not Work in the Same VLAN in Catalyst Switches
http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a008059a9df.shtml

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[Stephan Bahl · Accepted solution]

Hi Craig,

After we have a similar issue we doing some tests.

We find out the all Access Switches 2950, 2960-S, 3550, 3650 and 3750 are flooding the Multicast to every port in the Vlan, if there is no IGMP-querier in that Vlan.

Following configurations help to solve:

configure "ip igmp snooping querier address x.x.x.x"

               "ip igmp snooping querier"

or                                          

configure a SVI interface belongs to the vlan with "ip pim sparse-dense-mode" on one switch.

I prefer the SVI because here I can see more information’s with the sh ip igmp command.

After that IGMP-Snooping works and only on port where a client request the Mcast the Packets are forwarded.

Regards

Stephan

[craig.eyre]

Stephen,

This is exactly what we have seen here and just added PIM command to the upstream layer 3 interface.

Thanks for verifiying what we were seeing.

Craig