Matt
To setup multicasting across vlans involves only a few steps
1) enable multicast routing ie.
switch(config)# ip multicast-routing
2) enable multicast under each L3 interface that you have either
a) a multicast source OR
b) a multicast receiver
eg.
int vlan 20
ip pim dense-mode
Note that you can use either sparse mode or dense mode. If you use sparse mode additional config is needed but this is usually the recommended mode to use. To test basic multicast on a L3 switch LAN dense-mode is fine. Across a WAN you would definitely want to look at sparse-mode altho sparse-mode is also applicable to the LAN.
You then need a multicast application. As i said VLC is a good test tool. It allows you to stream an mpg across the network with multicast and then on a client you can run another copy of VLC to pick it up.
If you do use VLC and you are using multicast across vlans then make sure you set the TTL to more than 1. Each L3 hop will reduce the multicast TTL by 1.
IGMP which is used by hosts to register their interest in a multicast stream is enabled by default when you configure "ip pim sparse|dense mode" under the L3 vlan interface.
If you are not multicasting across vlans but merely within vlans then you may need to look at IGMP snooping if your switch supports it.
There is a fair bit of info above. Best thing to do is try and setup multicast in a lab environment with something like VLC and then come back to these forums if it isn't making sense :-)
Jon
