L2 or L3 multicast

[Pavel Dimow]

Hello,

is there any difference in terms of performance on 3750-X (or any other cisco switches) when running L2 versus L3 multicast?

The thing is that I can create either solution but I need to know of any cons if running either of those.

[Jon Marshall · Accepted solution]

Not sure what you mean when you say you can run either solution because the solution chosen depends on what you want to do ie. -

L2 multicast is concerned with delivering the multicast stream within a vlan eg. IGMP snooping etc.

L3 multicast is concerned with delivering the multicast stream between vlans eg PIM .

Obviously if you have L3 multicast routing you should probably also use L2 multicast. But if you only need to use multicast within a vlan then just use IGMP snooping plus the IGMP snooping querier function ie. no need for PIM.

Both L2 and L3 multicast are done in hardware on L3 switches. So it's not so much whether to use one or the other based on performance, it's what do you actually need to achieve.

Jon

[Joseph W. Doherty]

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Posting

As Jon has noted, both L2 or L3 should be processed in hardware, so there should be little performance difference in forwarding multicast packets.  That said, it would be an usual application that only used L2 mulitcast without also using L3 multicast.

Additionally, regarding performance there are a few things to keep in mind.  First, as also noted by Jon, if IGMP snooping isn't active (with a supporting querier), multicast will be flooded to all ports whether they want the stream or not.  Further, IGMP snooping, I believe, only works at L3, so if you did do L2 multicast only, you would flood all ports.

Remember blocks of L3 multicast addresses map into the same L2 Ethernet multicast address.  So, it's "better" to insure L3 multicast addresses map into different L2 multicast addresses.