Cisco NM-8B-U - HW ROUTERS L-M User Manual page 36

Feature Overview
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.
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.
Ethernet switch network modules 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.
Immediate-Leave Processing
IGMP snooping Immediate-Leave processing allows the switch to remove an interface that sends a leave
message from the forwarding table without first sending out MAC-based general queries to the interface.
The VLAN interface is pruned from the multicast tree for the multicast group specified in the original
leave message. Immediate-Leave processing ensures optimal bandwidth management for all hosts on a
switched network, even when multiple multicast groups are in use simultaneously.
You should use the Immediate-Leave processing feature only on VLANs where only one host is
Note
connected to each port. If Immediate-Leave is enabled on VLANs where more than one host is
connected to a port, some hosts might be inadvertently dropped. Immediate Leave is supported only
with IGMP version 2 hosts.
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:
• Snooping on PIM and DVMRP packets
• Statically connecting to a multicast router port with the ip igmp snooping mrouter global
configuration command
You can configure the switch to snoop on PIM/Distance Vector Multicast Routing Protocol
(PIM/DVMRP) packets. By default, the switch snoops on PIM/DVMRP packets on all VLANs. To learn
of multicast router ports through PIM-DVMRP packets, use the ip igmp snooping vlan vlan-id mrouter
learn pim-dvmrp interface command.
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
36
16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series