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Catalyst 3750 Metro Switch Software Configuration Guide
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Chapter 17 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Understanding IGMP Snooping
Layer 2 switches can use IGMP snooping to constrain the flooding of multicast traffic by dynamically
configuring Layer 2 interfaces so that multicast traffic is forwarded to only those interfaces associated
with IP multicast devices. As the name implies, IGMP snooping requires the LAN switch to snoop on
the IGMP transmissions between the host and the router and to keep track of multicast groups and
member ports. When the switch receives an IGMP report from a host for a particular multicast group,
the switch adds the host port number to the forwarding table entry; when it receives an IGMP Leave
Group message from a host, it removes the host port from the table entry. It also periodically deletes
entries if it does not receive IGMP membership reports from the multicast clients.
Note For more information on IP multicast and IGMP, refer to RFC 1112 and RFC 2236.
The multicast router (which could be a Catalyst 3750 Metro switch) sends out periodic general queries
to all VLANs. All hosts interested in this multicast traffic send join requests and are added to the
forwarding table entry. The switch creates one entry per VLAN in the IGMP snooping IP multicast
forwarding table for each group from which it receives an IGMP join request.
The switch supports IP multicast group-based bridging, rather than MAC-addressed based groups. With
multicast MAC address-based groups, if an IP address being configured translates (aliases) to a
previously configured MAC address or to any reserved multicast MAC addresses (in the range
224.0.0.xxx), the command fails. Because the switch uses IP multicast groups, there are no address
aliasing issues.
The IP multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure multicast groups by using the ip igmp snooping vlan vlan-id static ip_address interface
interface-id global configuration 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.
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.
These sections describe characteristics of IGMP snooping on the switch:
• Joining a Multicast Group, page 17-2
• Leaving a Multicast Group, page 17-4
• Immediate-Leave Processing, page 17-5
Joining a Multicast Group
When a host connected to the switch wants to join an IP multicast group, if it is an IGMP version 2 client,
it sends an unsolicited IGMP join message, specifying the IP multicast group to join. Alternatively, when
the switch receives a general query from the router, it forwards the query to all ports in the VLAN. IGMP
version 1 or version 2 hosts wanting to join the multicast group respond by sending a join message to
the switch. The switch CPU creates a multicast forwarding-table entry for the group if it is not already
present. The CPU also adds the interface where the join message was received to the forwarding-table
entry. The host associated with that interface receives multicast traffic for that multicast group.
See Figure 17-1.
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