From: Adam Crisp (adam.crisp@totalise.co.uk)
Date: Fri Nov 08 2002 - 13:25:09 GMT-3
cgmp / igmp snooping is all about reducing flooding on LANS.
If a (dumb) switch received a packet with a destinaion address that is a
multicast, then it is going to flood the packet - as if it were a broadcast.
this is because it doesn't know who the intended recevers are - ie what
ports the receivers are plugged into.
Cisco switches support two methods of getting round this problem.
1. CGMP
2. IGMP Snooping.
CGMP is cisco proprietary, you run it on the switch and on the router and
the router tells the switch what the receivers mac addresses are.
The router knows this because it has talked IGMP with the receiver.
The switch therefore doesn't need to flood the multicast packets and can
just forward it out of the correct ports.
IGMP snooping is where the catalyst listens to (snoops) the receivers and
routers talking IGMP and therefore already knows where the receiver are.
the 3550 supports igmp snooping - and it's on by default, so no you don't
need to do anything if using one of these.
If you're using another switch, check the documentation to find out whethe
it supports IGMP snooping - if not enable CGMP.
If you don't have cgmp enabled, and the switch doesn;t support igmp
snooping, then all that's going to happen is that the switch will forward
the multicast frames everyehere. It will still work.
good luck
Adam
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
jim.phillipo@guardent.com
Sent: 08 November 2002 16:03
To: ccielab@groupstudy.com
Subject: Multicast Question
Is CGMP required on a router interface if it is connected to a Catalyst
switch ?
And does it need enabling on the switch ?
I do not have it enabled and I am able to ping all four routers connected to
the same switch
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