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Catalyst 3560 Switch Software Configuration Guide
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Chapter 44 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
The Cisco IOS software supports these protocols to implement IP multicast routing:
Internet Group Management Protocol (IGMP) is used among hosts on a LAN and the routers (and
multilayer switches) on that LAN to track the multicast groups of which hosts are members.
Protocol-Independent Multicast (PIM) protocol is used among routers and multilayer switches to
track which multicast packets to forward to each other and to their directly connected LANs.
Distance Vector Multicast Routing Protocol (DVMRP) is used on the multicast backbone of the
Internet (MBONE). The software supports PIM-to-DVMRP interaction.
Cisco Group Management Protocol (CGMP) is used on Cisco routers and multilayer switches
connected to Layer 2 Catalyst switches to perform tasks similar to those performed by IGMP.
Figure 44-1 shows where these protocols operate within the IP multicast environment.
Figure 44-1 IP Multicast Routing Protocols
According to IPv4 multicast standards, the MAC destination multicast address begins with 0100:5e and
is appended by the last 23 bits of the IP address. On the Catalyst 3560 switch, if the multicast packet
does not match the switch multicast address, the packets are treated in this way:
If the packet has a multicast IP address and a unicast MAC address, the packet is forwarded in
software. This can occur because some protocols on legacy devices use unicast MAC addresses with
multicast IP addresses.
If the packet has a multicast IP address and an unmatched multicast MAC address, the packet is
dropped.
This section includes information about these topics:
Understanding IGMP, page 44-3
Understanding PIM, page 44-4
Understanding DVMRP, page 44-8
Understanding CGMP, page 44-9
Host
Host
PIM
IGMP
CGMP
DVMRP
Internet
MBONE
Cisco Catalyst switch
(CGMP client)
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