Posted on September 21, 2015 9:30 pm
 |  Asked by Shailendra Patil PATIL
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Can anyone explain me IgmpSnooping how it works in Mlag. I got the concept but I am unable to understand the querier, mrouter, igmphost etc..I am not able to understand how they all work and what is there importance. And I need to understand this in Mlag Scenario.

Please help me in understanding this.

Posted by John Gill
Answered on September 26, 2015 2:29 am


IGMP snooping is responsible for installing multicast mac addresses in the mac address table.  In MLAG, the unicast and multicast mac addresses are synchronized between the MLAG peers.  If a snooping entry exists on an MLAG port-channel, traffic destined to that group will be sent to that MLAG port-channel from either peer switch.  The decision of which switch to forward this traffic is made by an upstream device and its hashing.  Once traffic arrives on one of the MLAG peers, we will always try to forward it out a local interface, if possible.  Only once a mac address is learned on a non-MLAG port-channel or on a port-channel where the local interfaces are down, do we use the peer-link for forwarding.

Each switch will perform IGMP snooping somewhat independently, and only when they install a mac address will the synchronization occur.

The mrouter and querier function are operating at layer 3, and from that perspective, you can treat each MLAG peer as independent routers that will have one or more shared VLANs between each other.

Depending on where your queriers are, things can vary after this point.  Assuming you are running PIM on the MLAG pair, there are requirements in the 7124/48 and 7500 (gen1) switches do not meet, and we cannot run PIM+MLAG on these platforms.  Can you expand on the expected querier location and config, and if you were intending on running PIM and MLAG together?


Answered on September 29, 2015 3:43 am

Hi John,

Thanks for your info

In my case I have configured the querier on one of the Mlag( Ex. Mlag 20 ) and PIM router on Mlag 30. The Mlags here contain Eth Intfs from both the switches. Now when I send a UDP packet destined for a particular group( Ex. as it is a multicast packet it gets flodded onto the local switch. It doesnt go towards the peer-switch as the peer-link is not a part of the multicast group. When I disable the Mlags on one of the switch the Mlag status on both the switches become ”Inactive” Now the peer-link becomes a part of the multicast groups.

I need to understand why only in this case the peer-link becomes a part of Mlag????.. why not in the case when all the Mlags are active????.


Next, If the peer-link is part of the group and I send a UDP multicast packet from a local switch it gets flooded on to the local switch and as peer-link is part of the group It also gets received onto the peer-link which inturn travels to the peer switch. Now as in the local switch for Ex. Mlag 23 has received this packet will the Mlag 23 present onto the peer-switch also receives this????. Just clarify me on this scenario???

I want to know whether the peer-switches act independently in case of multicast scenario?????..

Please clarify me on the questions I asked above.

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