Design Reference

delivery or other large-scale, high-bandwidth multimedia applications. For instance, if you assign a
value that is too low, this can lead to a storm of membership reports if a large number of hosts are
subscribed. Similarly, assigning a value that is too high can cause unwanted high-bandwidth stream
propagation across the network if users change channels rapidly. Leave latency also depends on
the robustness value, so a value of 2 equates to a leave latency of twice the LMQI.
Determine the proper LMQI value for your particular network through testing. If a very large number
of users connect to a port, assigning a value of 3 can lead to a storm of report messages after a
group-specific query is sent. Conversely, if streams frequently start and stop in short intervals, as in
a TV delivery network, assigning a value of 10 can lead to frequent congestion in the core network.
Another performance-affecting factor that you need to be aware of is the error rate of the physical
medium. For links that have high packet loss, you can find it necessary to adjust the robustness
variable to a higher value to compensate for the possible loss of IGMP queries and reports.
In such cases, leave latency is adversely affected as numerous group-specific queries are
unanswered before the stream is pruned. The number of unanswered queries is equal to the
robustness variable (default 2). The assignment of a lower LMQI can counterbalance this effect.
However, if you configure the LMQI too low, it can actually exacerbate the problem by inducing
storms of reports on the network. LMQI values of 3 and 10, with a robustness value of 2, translate to
leave latencies of 6/10 of a second and 2 seconds, respectively.
When you choose an LMQI, consider all of these factors to determine the best configuration for the
given application and network. Test that value to ensure that it provides the best performance.
Important:
In networks that have only one user connected to each port, Avaya recommends that you use
the Fast Leave feature instead of LMQI, because no wait is required before the stream stops.
Similarly, the robustness variable does not affect the Fast Leave feature, which is an additional
benefit for links with high loss.
Multicast for multimedia
June 2015 Network Design Reference for Avaya VSP 4000 Series 147
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