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Hi all

Can anyone tell me what you need for Multicast?

am i right in saying if on the same network it will work? via IGMP.

and if it is needed to route the multicast between networks you need PIM etc ?

please explain the basic setup needed.



Cisco Employee

Re: Multicast


It depends whether IGMP snooping is enabled or not. If it is, you need to have a querier on the local subnet. It can either be a real router configured for multicast or a switch configured with the following command:

ip igmp snooping querier

This command will cause the switch to send IGMP queries on a regular interval. The interested workstations will respond with IGMP membership report message, which will keep IGMP snooping happy.


Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
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Cisco México 
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Super Bronze

Re: Multicast

At the most basic, all you need is a source to transmit mulitcast and receiver(s) on the same L2 "wire" (similar to a broadcast domain).

To jump to another "wire", you need some method to forward the multicast packets. This is usually accomplished by a multicast capable router that runs a mulitcast routing protocol.

PIM variants are multicast routing protocols that rely on an underlying unicast routing protocol.

IGMP is used to keep multicast routing routers apprised where the multicast receivers are. (Multicast source location is known because it's transmitting.)

(IGMP snooping switches examine IGMP packets between the IGMP querier [normally a multicast router] and multicast receivers to avoid sending multicast to ports that don't want it.)

Re: Multicast


I will try to simplify the concept for you:

"PIM" is a multicast routing protocol similar to "MOSPF" and "DVMRP".

PIM relies on the unicast routing table to build the multcast routing table, its also needed to forward multicast across layer-3 networks.

"Pim" uses the unicast routing table for the RPF check.

Multicast routing protocols like "DVMRP" and "MOSPF" uses Ships in the night techniqe which illustate independant multicast routing table.

"IGMP" is used for the communication between the hosts and routers.

IGMP has different versions 1,2 and 3.

each IGMP version has its own characterstics, enhancment made from 1 to 2 and between 2 and 3.

IGMP query message is sent by the router to each host on the subnet. You should have only one IGMP querier on a subnet.

IGMP membership report are sent by the host toward the router indicating current group activity.

IGMP membership update however, IGMPv2 is sent by the router to indicate Active hosts Group membership.

IGMPv3 includes the Source in the (*,G) message , so every host joins agroup with specific Source.

Hope this information answered your question,


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