Reaction To Topology Change Notifications - Cisco ASR 9000 Series Configuration Manual

IGMP Snooping Configuration at the Bridge Domain Level

Reaction to Topology Change Notifications

In a Spanning Tree Protocol (STP) topology, a Topology Change Notification (TCN) indicates that an STP
topology change has occurred. As a result of a topology change, mrouters and hosts reporting group membership
may migrate to other STP ports under the bridge domain. Mrouter and membership states must be relearned
after a TCN.
IGMP snooping reacts to TCNs in the following ways:
1 IGMP snooping temporarily extends the flood set for all known multicast routes to include all ports
participating in STP that are in forwarding state. The short-term flooding ensures that multicast delivery
continues to all mrouters and all member hosts in the bridge domain while mrouter and membership states
are relearned.
But, as a result of this TCN flooding, downstream STP links may sometimes become over-subscribed by
these extra multicast flows. This feature can in such cases be disabled by use of the tcn flood disable
command.
2 The STP root bridge issues a global leave ( for group 0.0.0.0) on all ports. This action triggers interoperable
IGMP queriers to send general queries, expediting the relearning process.
Note Sending global leaves for query solicitation is a Cisco-specific implementation.
3 When the TCN refresh period ends, IGMP snooping withdraws the non-mrouter and non-member STP
ports from the multicast route flood sets. You can control the amount of time that flooding occurs with
the tcn flood query count command. This command sets the number of IGMP general queries for which
the multicast traffic is flooded following a TCN, thereby influencing the refresh period.
IGMP snooping default behavior is that the STP root bridge always issues a global leave in response to a
TCN, and that the non-root bridges do not issue global leaves.
With the tcn query solicit command, you can enable a bridge to always issue a global leave in response to
TCNs, even when it is not the root bridge. In that case, the root bridge and the non-root bridge would issue
the global leave and both would solicit general queries in response to a TCN. Use the no form of the command
to turn off soliciting when the bridge is not the root.
One use for the tcn query solicit command is when Reverse Layer 2 Gateway Protocol (RL2GP) is
Note
configured to set up a MSTP Access Gateway. In this scenario, IGMP snooping is unaware of the root or
non-root status of the bridge and, therefore, when a TCN occurs, no query is solicited in the domain unless
IGMP snooping is explicity configured to do so on at least one bridge.
The root bridge always issues a global leave in response to a TCN. This behavior can not be disabled.
The internal querier has its own set of configuration options that control its reactions to TCNs.
The scope for all tcn related configuration option(s) is per bridge domain. If the command appears in profiles
attached to ports, it has no effect.
Cisco ASR 9000 Series Aggregation Services Router Multicast Configuration Guide, Release 6.0.x
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Implementing Layer-2 Multicast with IGMP Snooping