Posted on February 24, 2014 3:29 pm
 |  Asked by Chris
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I have seen several issues when we use multicast addresses on the same VLAN (not routed). For example with Mysql servers using a multicast IP for the heartbeat of the Checkpoint cluster control packet which use a multicast address for the synchronization, the servers do not seems to receive it correctly so we have to either switch to unicast or broadcast when possible.

Is there any restriction on the multicast IP we can use ?

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Posted by Chris
Answered on February 24, 2014 4:00 pm

Generally it is advised to avoid multicast groups with 0’s in the 2nd and 3rd octets (such as, as the associated multicast MAC addresses will alias with link local multicast groups, however this is not an absolute restriction.

One thing to keep in mind however is that by default IGMP snooping will be enabled on Arista switches.  Without an IGMP querier on the segment these tables will remain empty, and multicast traffic may be dropped.


With that in mind you have three options:

1) Disable IGMP snooping and allow traffic to be flooded within the VLAN:

-No IP IGMP snooping

2) Create static IGMP snooping entries for your multicast groups, this will avoid the flooding but can be laborious for a large number of groups:

-IP IGMP snooping VLAN <VLAN> static <IP address> interface <list of interfaces which should receive this traffic>

3)Enable the IGMP snooping querier.  This feature will program the IGMP snooping tables, even if there is no layer 3 interface configured on the Arista switch.

-IP IGMP snooping VLAN <VLAN> querier address <IP Address>

-IP IGMP snooping VLAN <VLAN> querier

Please note, the above method requires that your multicast receivers are sending IGMP joins.

Posted by Ben DeBolle
Answered on February 27, 2015 12:07 am

The #2 option above is not sufficient alone.  IGMP Snooping will only prune traffic if there is a querier on the VLAN.  Without a querier the switch can’t tell if there is any listener on a port, so it will still flood. Static entries are not to suppress flooding but to forward even when there is no response to queries.

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