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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information learned by switch may be out of date.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.
If a topology change notification (TCN) is received, and all the uplink ports are
subsequently deleted, a time out mechanism is used to delete all of the
currently learned multicast channels.
When a new uplink port starts up, the switch sends unsolicited reports for all
currently learned channels out the new uplink port.
By default, the switch immediately enters into "multicast flooding mode" when
a spanning tree topology change occurs. In this mode, multicast traffic will be
flooded to all VLAN ports. If many ports have subscribed to different multicast
groups, flooding may cause excessive packet loss on the link between the
switch and the end host. Flooding may be disabled to avoid this, causing
multicast traffic to be delivered only to those ports on which multicast group
members have been learned. Otherwise, the time spent in flooding mode can
be manually configured to reduce excessive loading.
When the spanning tree topology changes, the root bridge sends a proxy
query to quickly re-learn the host membership/port relations for multicast
channels. The root bridge also sends an unsolicited Multicast Router Discover
(MRD) request to quickly locate the multicast routers in this VLAN.
The proxy query and unsolicited MRD request are flooded to all VLAN ports
except for the receiving port when the switch receives such packets.
◆
TCN Query Solicit – Sends out an IGMP general query solicitation when a
spanning tree topology change notification (TCN) occurs. (Default: Disabled)
When the root bridge in a spanning tree receives a TCN for a VLAN where IGMP
snooping is enabled, it issues a global IGMP leave message (or query
solicitation). When a switch receives this solicitation, it floods it to all ports in
the VLAN where the spanning tree change occurred. When an upstream
– 533 –
Chapter 16
| Multicast Filtering
Layer 2 IGMP (Snooping and Query)
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