IGMP Querier on switch, SVI on upstream device

[mrjdh]

Hi all,

I have a question that I can't find the answer to. 

When an IGMP querier is required to support IGMP snooping for hosts in the same VLAN, if the SVI/gateway is positioned upstream from the switch where the hosts are connected, is this where the querier needs to be configured, or could it also be configured on the switch itself (no SVI/gateway setup for that VLAN)? Would the answer apply to both Catalyst and Nexus product lines?

Also, when a querier periodically fires out the queries, what address does it send the query to? A generic multicast address, or to the address of the chosen multicast group IP?

Thanks. 

[Torbjørn]

The gateway device must be an IGMP querier to maintain group subscribers on the VLAN.  You can configure your switch as a secondary querier as well. By doing this you'll be able to support IGMP snooping in absence of your gateway device. You can read more about the IGMP snooping querier function here: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/37e/multicast/configuration_guide/b_mc_37e_3850_cg/b_mc_3e_3850_cg_chapter_0100.pdf

The addresses used varies by IGMP versions and message types.

For IGMPv2 the following messages and addresses are used:

• Membership query - general queries are sent to 224.0.0.1(all hosts) and group specific queries are sent to the group address.
• Membership report - sent to 224.0.0.2(all routers) if unsolicited, but to the group address if it is sent in response to a query.
• Leave group - sent to 224.0.0.2 to leave a group.

For IGMPv3 the address 224.0.0.22 is used instead of the all hosts multicast address in join(membership report) and leave messages.

[Joseph W. Doherty]

If multicast routing is in use, the multicast router(s) (or L3 switches), for a L2 segment, handle IGMP.  I.e. it's automatic (because multicast routing needs to know.)

I.e. the only time a L2 switch needs to also perform IGMP is if, as you noted, you want to use IGMP snooping, when you don't have a multicast routing interface on that L2 segment.

[paul driver]

Hello
If the mc scr/recv hosts are all same vlan , then Igmp snooping needs to be enabled to the vlan on all switches where these hosts recvs are located

Snooping querier to be applied on the core switch for that vlan which then should provide Mcast to all your lan hosts