Users Guide

928 Layer-2 Multicast Features
Group addresses that fall into the reserved range 224.0.0.x are never pruned
by IGMP snooping—they are always flooded to all ports in the VLAN. Note
that this flooding is based on the IP address, not the corresponding 01-00-5e-
00-00-xx MAC address.
When a multicast router is discovered (or locally configured on the switch),
its interface is added to the interface distribution list for all multicast groups
in the VLAN. If a switch is connected to a multicast source and no client, the
switch filters the traffic from that group to all interfaces in the VLAN. If the
switch sees an IGMP join from a host in the same VLAN, then it forwards the
traffic to the host. Likewise, if the switch sees a multicast router in the VLAN,
it forwards the group to the multicast router and does not flood in the VLAN.
If snooping is disabled, the switch always floods multicast data and control
plane packets in the VLAN.
A multicast router can also be statically configured, either by configuring a
port as a static L2 mrouter port in the VLAN, or by enabling L3 multicast
routing in the switch. If a port is configured as a static L2 mrouter port, IGMP
snooping forwards multicast data plane packets in the VLAN regardless of the
interface state of the port.
By default, dynamically discovered multicast routers are aged out every five
minutes. The user can control whether or not multicast routers age out. If all
multicast routers age out, the switch floods the VLAN with any received
multicast groups.
Multicast routers send an IGMP query every 60 seconds. This query is
intercepted by the switch and forwarded to all ports in the VLAN. All hosts
that are members of the group answer that query. The switch intercepts the
replies and forwards only one report per group from all of the received
responses.
In summary:
• IGMP snooping controls the flooding/forwarding behavior for multicast
groups. Multicast data is flooded in the VLAN until a multicast router port
is identified.
• IGMP snooping is enabled by default
• IGMP snooping forwards multicast sources to multicast routers by default
• Reserved multicast IP addresses (224.0.0.x) are always flooded to all ports
in the VLAN