IGMP Snooping Querier switch (w/o receivers) receiving multicast traffic

[Qunli Zhang]

Dear networkers,

For a pretty simple setup, all based on VLAN100, SW1 runs as IGMP snooping querier on VLAN100, SW1 simply sends the IGMP query commands, it's no necessary for it to get the real multicast data traffic as there is no any receiver, can anyone explain this? thanks.

[p.kellett]

Dear Qunli Zhang,

IGMP snooping allows the switch to keep track of receivers on particular ports who are requesting multicast traffic.

Normally, a multicast router acts as an IGMP querier and sends queries every 60 seconds to the 224.0.0.1 address (All systems on this subnet). By getting responses back from receivers, the switch can keep track of which receivers are still active.

In a situation where there is no multicast router, an IGMP Snooping querier can be run. This querier will send out the queries so that receiver information is kept up to date.

Thanks,

Paul Kellett

[playerplease]

But is the IGMP snooping querier supposed to send PIM messages ? 

if yes , then it's normal that SW2 forward known and unknown multicast traffic to its mrouter port (g0/2), 

but to me , the "snooping querier" only sends IGMP general queries 224.0.0.1 , no PIM messages ( you don't configure "Ip pim.." on the VLAN interface of SW1?) so there shouldn't be any mrouter port on SW2, therefore unknown multicast shouldn't  go on g0/2.

Any opinion on that ?

[paul driver]

Hello

I agree without pim (L3) being enabled or the querirer (L2), Then Sw1 will just absorb the host report and switch 2 wouldn't have any clue of the receiver on switch 1.

res
Paul

[willwetherman]

Hello,

Correct, a switch operating as an IGMP querier will only send IGMP general query messages and will not send any PIM messages. However, it are these IGMP general query messages that SW2's IGMP snooping process uses to determine that Gig0/2 is an mrouter port.

See the following link

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/68131-cat-multicast-prob.html#understand

Solution 2: Enable IGMP Querier Feature on a Layer 2 Catalyst Switch

The IGMP querier is a relatively new feature on Layer 2 switches. When a network/VLAN does not have a router that can take on the multicast router role and provide the mrouter discovery on the switches, you can turn on the IGMP querier feature. The feature allows the Layer 2 switch to proxy for a multicast router and send out periodic IGMP queries in that network. This action causes the switch to consider itself an mrouter port. The remaining switches in the network simply define their respective mrouter ports as the interface on which they received this IGMP query.

[willwetherman · Accepted solution]

Hi,

If I understand your query correctly, you want to understand why multicast traffic is sent to both the receiver that is directly connected to SW2 and to SW1 which doesn’t have any receivers. Is that correct?

If so this is expected behaviour as SW2's IGMP snooping process will automatically discover switchports (Gig0/2 in this instance) that are connected to a multicast capable router/switch by listening to the PIM and IGMP messages. Thinking that a multicast capable device is connected to Gig0/2, SW2 will flood all received multicast traffic out of this port so that it can reach other potential receivers further into the network. SW2 isn’t aware that SW1 is acting as just an IGMP querier and is not running PIM.

If you issue ‘show ip igmp snooping mrouter’ on SW2 you should see an output that looks as follows

SW2#show ip igmp snooping mrouter

Vlan    ports

----    -----

 100    Gig0/2(dynamic)

[Qunli Zhang]

Thanks willwetherman, that makes a lot sense!