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New Member

muticasting in a single switch


Question is on the cat6500 with sup2 and msfc2.

Assuming that there is just single cat6500 and it has 50xFast etherenet ports, and configured few vlans for unicast and broadcast.

vlan1: port1-25

vlan2: port26-50

And configured the 10 multicast groups, one group per 10 ports (i.e. there are 5 multicast groups in a cat6500).

port01-10: group1

port11-20: group2


port41-50: group5

To make this configuration, I don't think we need to enable any multicast routing protocols such as PIM since it doesn't require exchanging group information between routers (i.e. single chassis). Just need to enable the IGMP on the IOS to run the multicasting in each group.

My theory is correct?

In other words, the multicasting should work by default though we don't do any configuration for the multicasting since the igmp is enabled by default.

Thx in advance,



Re: muticasting in a single switch

The theory is not correct.

If you are routing multicast traffic between vlans, then you must enable multicast routing , which involves enabling PIM.

IGMP Snooping is needed so that the switch knows exactly which switch port to forward the multicast traffic to.

I haven't found any good URL on that gives a good primer on multicast and issues to consider when designing multicast.

Here are some links that may be useful to just understand how to configure multicast routing and IGMP Snooping/CGMP on cisco L3 switches.



I'd suggest that you buy the Cisco Press book called "Developing IP multicast networks"

by Beau Williamson. Its a great book for folks who are new to multicast and want to deploy it in their network properly.

New Member

Re: muticasting in a single switch

Thanks skarundi for correction and good book recommendation (just placed order).

Another assuming that if we use the one vlan and 5 multicast groups, then we don't need the multicast routing, just need to enable the igmp(L3) and igmp snooping(L2), right?

Also wondering, in order to enable the IGMP(L3), do we have to enable the multicast routing? i.e. no support command to enable the igmp only, right?

If you give me one more reply, it would be appreciated.