Igmp snooping and recovery times, Health message priority – Allied Telesis AlliedWare OS User Manual
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Page 31 | AlliedWare™ OS How To Note: EPSR
IGMP Snooping and Recovery Times
IGMP Snooping and Recovery Times
Since Software Version 281-03, IGMP snooping includes query solicitation, a new feature
that minimises loss of multicast data after a topology change.
When IGMP snooping is enabled on a VLAN, and EPSR changes the underlying link layer
topology of that VLAN, this can interrupt multicast data flow for a significant length of time.
Query solicitation prevents this by monitoring the VLAN for any topology changes. When it
detects a change, it generates a special IGMP Leave message known as a Query Solicit, and
floods the Query Solicit message to all ports. When the IGMP Querier receives the message,
it responds by sending a General Query. This refreshes snooped group membership
information in the network.
Query solicitation functions by default (without you enabling it) on the EPSR master node. By
default, the master node always sends a Query Solicit message when the topology changes.
On other switches in the network, the query solicitation is disabled by default, but you can
enable it by using the command:
set igmpsnooping vlan={vlan-name|1..4094|all}
querysolicit={on|yes|true}
If you enable query solicitation on an EPSR transit node, both that node and the master node
send a Query Solicit message.
Once the Querier receives the Query Solicit message, it sends out a General Query and
waits for responses, which update the snooping information throughout the network. If
necessary, you can reduce the time this takes by tuning the IGMP timers, especially the
queryresponseinterval parameter. For more information, see the “IGMP Timers and
Counters” section of “How To Configure IGMP on Allied Telesyn Routers and Switches for
Multicasting”. This How To Note is available in the Resource Center of the Documentation
and Tools CDROM for Software Version 2.8.1, or from:
Query solicitation also works with networks that use Spanning Tree (STP, RSTP, or MSTP).
Health Message Priority
EPSR uses Health messages to check that the ring is intact. If switches in the ring were to
drop Health messages, this could make the ring unstable. Therefore, Health messages are
sent to the highest priority queue (queue 7), which uses strict priority scheduling by default.
This makes sure that the switches forward Health messages even if the network is congested.
We recommend that you leave queue 7 as the highest priority queue, leave it using strict
priority scheduling, and only send essential control traffic to it.
In the unlikely event that this is impossible, you can increase the failover time so that the
master node only changes the ring topology if several Health messages in a row fail to arrive.
By default, the failover time is set to two seconds, which means that the master node decides
that the ring is down if two Health messages in a row fail to arrive.