Joining A Multicast Group; Chapter 16 Configuring Igmp Snooping And Mvr - Cisco Catalyst 2950 Software Configuration Manual

Chapter 16 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
host, it removes the host port from the table entry. After it relays the IGMP queries from the multicast
router, it deletes entries periodically if it does not receive any IGMP membership reports from the
multicast clients.
When IGMP snooping is enabled, the multicast router sends out periodic IGMP general queries to all
VLANs. The switch responds to the router queries with only one join request per MAC multicast group,
and the switch creates one entry per VLAN in the Layer 2 forwarding table for each MAC group from
which it receives an IGMP join request. All hosts interested in this multicast traffic send join requests
and are added to the forwarding table entry.
Layer 2 multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure MAC multicast groups by using the ip igmp snooping vlan static command. If you specify
group membership for a multicast group address statically, your setting supersedes any automatic
manipulation by IGMP snooping. Multicast group membership lists can consist of both user-defined and
IGMP snooping-learned settings.
Catalyst 2950 switches support a maximum of 255 IP multicast groups and support both IGMP version 1
and IGMP version 2.
If a port spanning-tree, a port group, or a VLAN ID change occurs, the IGMP snooping-learned multicast
groups from this port on the VLAN are deleted.
In the IP multicast-source-only environment, the switch learns the IP multicast group from the IP
multicast data stream and only forwards traffic to the multicast router ports.

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.
See Figure 16-1. Host 1 wants to join multicast group 224.1.2.3 and multicasts an unsolicited IGMP
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 Table 16-1 that includes the port numbers of Host 1 and the router.
Catalyst 2950 Desktop Switch Software Configuration Guide
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