Unanswered Question
Sep 4th, 2010


What is IGMP and CGMP??

What is PIM and MIB??

I have this problem too.
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Giuseppe Larosa Sun, 09/05/2010 - 00:20

Hello Abnihay,

IGMP is a protocol used between hosts and routers to communicate interest about multicast sessions (groups)

IGMP version 1-2 defines a multicast session only with the multicast address.

IGMP version 3 defines a multicast session by looking at multicast destination address and source (S,G) and it is needed for Source Specific Multicast PIM SSM.

PIM = protocol indipendent multicast is the multicast routing protocol spoken between multicast capable routers end to end between multicast sources and multicast receivers.

PIM is indipendent for this aspect: it uses every information in the unicast routing protocol to decide if a multicast packet (S,G) received on an interface has to forwarded or dropped. PIM uses the unicast routing table to perform reverse path forwarding that is the multicast packet has been received on the same interface the router would use to reach source S or an another interface?.

The objective is to forward with the minimum number of stream replications in order to avoid to waste bandwidth.

PIM version 2 is the current version and can work in the following modes: PIM DM (dense mode), PIM SM ( Also called ASM), PIM SSM or PIM bidirectional. To be noted different PIM modes can exist at the same time on a network as the decision of what mode to use is taken from the group address G: the  address space can be divided and you can have a space used by PIM ASM, another by PIM SSM and another one for PIM Bidir.

This is part of the group mapping.

CGMP=  an old OSI L2 protocol used between multicast routers and L2 LAN switches with the objective to optimize multicast forwarding over ports of the switches.

Now the current tecnique used is IGMP snooping performed by switches using ASIC dedicated hardware without involving the multicast routers.

see for an introduction to multicast


MIB = SNMP data structure

Hope to help


Nagaraja Thanthry Sun, 09/05/2010 - 08:20


IGMP is a Multicast related protocol used to register clients in a LAN

environment. Using IGMP, the Multicast enabled router keeps track of all the

active clients in the network behind it.



CGMP is Cisco Group Management Protocol developed for Catalyst Switches.

Since Switches cannot process L3 packets, and hence they will not know the

active clients in a multicast group, CGMP allows these switches to talk to

the multicast enabled routers and get the information. This helps the

switches in making better forwarding decisions.



PIM is Protocol Independent Multicast routing protocol. It is used to build

multicast trees.



MIB is Management Information Base used to define the SNMP templates.



Hope this helps.




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