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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 24 Configuring IP Multicast Routing
Cisco Implementation of IP Multicast Routing
CGMP is a protocol used on Cisco routers and multilayer switches connected to Layer 2 Catalyst
switches to perform tasks similar to those performed by IGMP. CGMP permits Layer 2 group
membership information to be communicated from the CGMP server to the switch, which can learn on
which ports multicast members reside instead of flooding multicast traffic to all switch ports. (IGMP
snooping is another method to constrain the flooding of multicast packets. For more information, see
Chapter 11, “Configuring IGMP Snooping and MVR.”
)
CGMP is necessary because the Layer 2 switch cannot distinguish between IP multicast data packets and
IGMP report messages, which are both at the MAC-level and are addressed to the same group address.
Joining a Group with CGMP
Hosts connected to a Layer 2 Catalyst switch can join a multicast group by sending an unsolicited IGMP
membership report message to the target group (224.1.2.3) as shown in
Figure 24-7
. Because LAN
switches operate at Layer 2 and understand only MAC addresses, the source and destination fields of the
frame contain 48-bit MAC addresses for Host 3 (0080.c7a2.1093) and MAC-address equivalent of the
multicast group address (0100.5e01.0203).
The IGMP membership report is received by the Layer 2 switch and forwarded to the CGMP server for
normal IGMP processing. The CGMP server, which must have CGMP enabled on the interface
connected to the Layer 2 switch, receives the membership report and translates the report into a CGMP
join message. It sends the CGMP join message to the switch through the well-known CGMP multicast
MAC address (0x0100.0cdd.dddd). When the Layer 2 switch receives the join message, it updates its
forwarding table to include the MAC-equivalent of the group destination address and the applicable
input and output switch ports.
Figure 24-7 Host Joining a Group Using CGMP
IGMP Membership Report
Cisco router or
Catalyst 3550 multilayer switch
(CGMP server)
Host 1
Source group
224.1.2.3
Host 2
Host 4
Host 3
Receiver
CGMP is enabled on this interface.
Catalyst Layer 2 switch (CGMP client)
CGMP Join Message
45150
Dest MAC = 0100.5e01.0203
Source MAC = 0080.c7a2.1093
Group = 224.1.2.3
Source = 0080.c7a2.1093
Group Dest = 0100.5e01.0203
