Chapter 9: Multicast 143
Section 9-2
■ Multicast routers cannot keep a detailed list of all hosts belonging to a multicast group.
Rather, a router knows only which multicast groups are active on specific subnets.
■ The switch also relays the initial join request for a multicast group to all its known
multicast routers.
■ If no multicast routers are present in the network, the switch can be configured to
act as an IGMP querier.
■ When a host wants to leave a multicast group, IGMPv1 detects only the absence of
its membership reports. IGMPv2, however, allows the host to send an IGMP leave
group message to the “all-routers” multicast group 224.0.0.2 at any time.
■ When the switch intercepts an IGMP leave group message on a switch port, it nor-
mally sends a query to that multicast group back out the same switch port. If no
hosts respond to the query and no multicast routers have been discovered on the
switch port, that port is removed from the multicast group. IGMP Fast-Leave
Processing can be used to allow the switch to immediately remove a port from a
multicast group after a Leave Group message is received.
■ Spanning tree topology changes that occur in a VLAN also cause the switch to purge
any multicast group information learned through IGMP snooping. That information
must be relearned.
Configuration
1. (Optional) Enable IGMP snooping:
(interface) ip igmp snooping
IGMP snooping is enabled by default. Use the no form of this command to disable
IGMP.
2. (Optional) Enable IGMP on a per VLAN basis:
(interface) ip igmp snooping vlan vlan-id
IGMP snooping is enabled on every VLAN. By disabling IGMP globally with the no
igmp snooping command, you can enable IGMP on a per VLAN basis with this
command.
3. (Optional) Allow snooping to learn from another source:
(interface) ip igmp snooping mrouter learn {cgmp | pim-dvmrp}
In addition to normal IGMP snooping, the switch can also learn by listening to
CGMP messages (cgmp) or PIM-DVMRP messages (pim-dvmrp).
4. (Optional) Use IGMP Fast-Leave Processing:
(interface) ip igmp snooping fast-leave
By default, Fast-Leave Processing is disabled. Fast-Leave improves the latency of
multicast group removal but should be used only on VLANs where single hosts con-
nect to each switch port.
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