Question regarding IGMP snooping

[d-fillmore]

Hi, I have 10 servers connected to a VLAN that communicate with each other using multicast. They do this through a 6509 w/SUP720s. The switch just provides L2 connectivity between the hosts, it does not have multicast routing enabled.

The problem I have is that these servers cannot communicate with each other unless IGMP snooping is turned off globally. This confuses me because although IGMP snooping is globally enabled by default, there is no multicast routing configured and thus there is no configured IGMP snooping querier configured.

Ideally I don't want to disable IGMP snooping globally, as it would be useful should the servers start sending multicast traffic over some of their other interfaces.

My understanding of IGMP snooping is that it needs a querier to work, yet in my case, it seems to be doing something without having a querier configured.

Can anyone offer any advice on this?

Many Thanks in advance

Dom

[Giuseppe Larosa]

Hello Dom,

your understanding is correct.

IGMP snooping is thought to optimize multicast forwarding but if no igmp queries and igmp reports can be seen on the Vlan the effect is to empty the list of ports in the vlan that can forward multicast frames for a given group.

You can configure an igmp snooping querier on the vlan for your scenario

see the following link

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/snooigmp.html#wp1070778

Configure one switch as the IGMP snooping querier in each VLAN that is supported on switches that use IGMP to report interest in IP multicast traffic.

only advice it looks like to support IGMPv3 queries so verify if your servers can answer to them.

Hope to help

Giuseppe

[d-fillmore]

Hi Giuseppe,

Many Thanks for your reply.

So if a switch is to pass multicast traffic just at layer 2, then IGMP snooping needs to be turned off globally, then re-enabled for the other vlans by configuring IGMP queriers?

Many Thanks in advance

Dom

[Giuseppe Larosa · Accepted solution]

Hello Dom,

my understanding is that you can define an IGMP snooping querier on the vlan of the servers without disabling IGMP snooping globally or in the server Vlan.

By setting an IGMP snooping querier you have a device (it can be the switch itself) that sends IGMP queries on the VLAN even without be configured with ip pim sparse-mode.

In this way all the devices that will answer to the periodic queries will be added to the list of ports in the vlan for that multicast group (think of it as a L2 oilist )

It is just a way to have IGMP snooping to make its job that is to intercept IGMP reports from receivers when no multicast router is present in the Vlan/broadcast domain.

For a router is enough to get an answer to keep the interface in the oilist for the group (last reporter)

The switch needs to track all the ports where an answer to the IGMP query is received to build the L2 oilist for forwarding optimization.

Hope to help

Giuseppe

[d-fillmore]

Mny Thanks for your time Giuseppe