for IGMP Snooping, it is enable by default. If you are referring to ip multicast routing (L3 multicast) it's as follows:
conf t
ip muticast-routing
interface vlan x
ip pim sparse-dense mode
or
ip pim sparse mode
or
ip pim dense mode
Q. What is Sparse-Dense?
A. Sparse-Dense is an interface configuration that allows both DM groups and SM groups to flow across that interface. It is normally only applied when Auto-RP is used to send RP information.
This link is intorduction to multicast presentatoin, the same presentation given at networkers:
http://www.cisco.com/application/pdf/en/us/guest/products/ps6552/c1161/cdccont_0900aecd80310883.pdf
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