CHAPTER
11-1
Catalyst 2950 Desktop Switch Software Configuration Guide
78-11380-03
11
Configuring IGMP Snooping and MVR
This chapter describes how to configure Internet Group Management Protocol (IGMP) snooping on your
switch, including an application of local IGMP snooping, Multicast VLAN Registration (MVR).
Note For complete syntax and usage information for the commands used in this chapter, refer to the
Catalyst 2950 Desktop Switch Command Reference for this release and the Cisco IOS Release Network
Protocols Command Reference, Part 1, for Release 12.1.
This chapter consists of these sections:
• Understanding and Configuring IGMP Snooping, page 11-1
• Understanding Multicast VLAN Registration, page 11-7
Note For MAC addresses that map to IP multicast groups, you can either manage them through features such
as IGMP snooping and MVR, or you can use static MAC addresses. However, you cannot use both
methods simultaneously. Therefore, before using IGMP snooping or MVR, you should remove all
statically configured MAC addresses that map to IP multicast groups.
Understanding and Configuring IGMP Snooping
Internet Group Management Protocol (IGMP) snooping constrains the flooding of multicast traffic by
dynamically configuring the interfaces so that multicast traffic is forwarded only to those interfaces
associated with IP multicast devices. The LAN switch snoops on the IGMP traffic between the host and
the router and keeps track of multicast groups and member ports. When the switch receives an IGMP
join report from a host for a particular multicast group, the switch adds the host port number to the
associated multicast forwarding table entry. When it receives an IGMP Leave Group message from a
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.
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