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Page 33 | AlliedWare™ OS How To Note: IGMP
Query solicitation (rapid recovery from topology changes) > Speeding up IGMP convergence in a non-looped topology
Speeding up IGMP convergence in a non-looped
topology
The previous section described how it can take up to 125 seconds for multicasting to recover
in a non-looped topology after a port comes back up. You can speed up convergence simply
by enabling RSTP. This enables the network to use Query Solicitation and means that
multicasting resumes within 3 seconds of the link coming up.
Even though there is no loop in the network, one of the switches becomes the STP root
bridge—it does not matter which switch does this. When the link comes up, the root bridge
detects the topology change and sends a Query Solicitation.
So you just need to enter the following commands on all switches:
enable stp=default
set stp=default mode=rapid
Enabling query solicitation on multiple switches in a
looped topology
On networks that use spanning tree or EPSR, Query Solicitation is not normally required on
switches other than the STP root bridge or EPSR master node. Therefore, it is only enabled
by default on the root bridge and the master node.
However, in some networks you may need to turn on Query Solicitation on all switches—for
example, if the network includes other switches that do not support Query Solicitation and
therefore the STP root bridge may be a switch that does not send Query Solicit messages. To
enable Query Solicitation, use the command:
set igmpsnooping vlan={vlan-name|1..4094|all} querysolicit={on|
yes|true}
Every switch that has Query Solicitation enabled sends a Query Solicit message when it
detects a topology change. Enabling it on multiple switches means you get multiple messages,
but has no other disadvantage.
The following figure shows a the packet flow for a four-switch network with Query
Solicitation enabled on all the switches.