IGMP Snooping

IGMP Snooping
While CGMP is a Cisco proprietary protocol to enable switches and routers to communicate
regarding multicast traffic patterns, IGMP Snooping is referenced in IGMPv3 and does that
same thing. Several vendors have created implementations of IGMP Snooping that don’t quite
play well with each other.
IGMP Snooping doesn’t require any sort of translation into a different protocol at the switch.
IGMP is used from the client to the router. The switch monitors, or sniffs, the IGMP packets as
they pass through and records the MAC addresses and the port that requested to be a part of
the process.
Host A sends
IGMP report for
224.2.165.145.
Router receives report,
creates GDA and USA,
and enables multicast
forwarding on interface FE0/0.
Host B
Host A
Host C
Router 1 Switch 1
1
2
2
1
3
Switch receives CGMP Join.
Establishes a multicast CAM
entry for port 2/1.
3
2/1
V1 2/2
2/3
FE0/0
604 Chapter 19  Understanding and Configuring Multicast Operation
Because the switch becomes an integral part of the process of IGMP, the router forwards status
messages to the switch and the switch forwards them out the appropriate ports. This is the
process of Fast-Leave and is done on both CGMP and IGMP Snooping:
 Client A is listening to a multicast stream and decides to stop listening. The client sends an
IGMP Leave message to the switch.
 The switch responds with an IGMP Query to find out whether other clients exist that still
want that multicast stream.
 If a client exists out that port, the switch makes no changes.
 If there is no reply out that port but other ports are receiving the stream, the switch
does nothing.
 If there is no reply to the Query and there are no other ports participating, the switch
forwards the Leave message to the router.