Chapter 14
| Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
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◆ IGMP Querier – A router, or multicast-enabled switch, can periodically ask their
hosts if they want to receive multicast traffic. If there is more than one router/
switch on the LAN performing IP multicasting, one of these devices is elected
“querier” and assumes the role of querying the LAN for group members. It then
propagates the service requests on to any upstream multicast switch/router to
ensure that it will continue to receive the multicast service.
Note:
Multicast routers use this information from IGMP snooping and query
reports, along with a multicast routing protocol such as DVMRP or PIM, to support
IP multicasting across the Internet.
Parameters
These parameters are displayed:
◆ IGMP Snooping Status – When enabled, the switch will monitor network
traffic to determine which hosts want to receive multicast traffic. This is referred
to as IGMP Snooping. (Default: Enabled)
When IGMP snooping is enabled globally, the per VLAN interface settings for
IGMP snooping take precedence (see “Setting IGMP Snooping Status
per Interface” on page 550).
When IGMP snooping is disabled globally, snooping can still be configured per
VLAN interface, but the interface settings will not take effect until snooping is
re-enabled globally.
◆ Proxy Reporting Status – Enables IGMP Snooping with Proxy Reporting.
(Default: Disabled)
When proxy reporting is enabled with this command, the switch performs
“IGMP Snooping with Proxy Reporting” (as defined in DSL Forum TR-101, April
2006), including last leave, and query suppression.
Last leave sends out a proxy query when the last member leaves a multicast
group, and query suppression means that specific queries are not forwarded
from an upstream multicast router to hosts downstream from this device.
When proxy reporting is disabled, all IGMP reports received by the switch are
forwarded natively to the upstream multicast routers.
◆ TCN Flood – Enables flooding of multicast traffic if a spanning tree topology
change notification (TCN) occurs. (Default: Disabled)
When a spanning tree topology change occurs, the multicast membership
information learned by switch may be out of date. For example, a host linked to
one port before the topology change (TC) may be moved to another port after
the change. To ensure that multicast data is delivered to all receivers, by
default, a switch in a VLAN (with IGMP snooping enabled) that receives a Bridge
Protocol Data Unit (BPDU) with TC bit set (by the root bridge) will enter into
“multicast flooding mode” for a period of time until the topology has stabilized
and the new locations of all multicast receivers are learned.
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