Setting The Snooping Method; Joining A Multicast Group - Cisco Catalyst 2950 Software Manual

Understanding and Configuring IGMP Snooping

Setting the Snooping Method

Multicast-capable router ports are added to the forwarding table for every IP multicast entry. The switch
learns of such ports through one of these methods:
•
•
•
You can configure the switch to either snoop on Protocol Independent Multicast/Distance Vector
Multicast Routing Protocol (PIM/DVMRP) packets or to listen to CGMP self-join packets. By default,
the switch snoops on PIM/DVMRP packets on all VLANs. To learn of multicast router ports through
only CGMP self-join packets, use the ip igmp snooping vlan vlan-id mrouter learn cgmp global
configuration command. When this command is used, the router listens only to CGMP self-join packets
and no other CGMP packets. To learn of multicast router ports through only PIM-DVMRP packets, use
the ip igmp snooping vlan vlan-id mrouter learn pim-dvmrp interface command.

Joining a Multicast Group

When a host connected to the switch wants to join an IP multicast group, it sends an IGMP join message,
specifying the IP multicast group it wants to join. When the switch receives this message, it adds the port
to the IP multicast group port address entry in the forwarding table.
Refer to
membership report (IGMP join message) to the group with the equivalent MAC destination address of
0100.5E01.0203. The switch recognizes IGMP packets and forwards them to the CPU. When the CPU
receives the IGMP report multicast by Host 1, the CPU uses the information to set up a multicast
forwarding table entry as shown in
Figure 11-1 Initial IGMP Join Message
Catalyst 2950 Desktop Switch Software Configuration Guide
11-4
Snooping on PIM and DVMRP packets
Listening to CGMP self-join packets from other routers
Statically connecting to a multicast router port with the ip igmp snooping mrouter global
configuration command
Figure 11-1. Host 1 wants to join multicast group 224.1.2.3 and multicasts an unsolicited IGMP
Catalyst 2950 switch
CPU
0
CAM
Table
2
Host 1 Host 2
Table 11-1 that includes the port numbers of Host 1 and the router.
Router A
1
IGMP Report 224.1.2.3
3 4 5
Host 3 Host 4
Chapter 11 Configuring IGMP Snooping and MVR
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