Fast Leave; Last Member Query Interval Tuning - Avaya 8800 Planning And Engineering, Network Design

Fast Leave

IGMP Fast Leave supports two modes of operation: Single User Mode and Multiple User Mode.
In Single User Mode, if more than one member of a group is on the port and one of the group
members leaves the group, everyone stops receiving traffic for this group. A Group-Specific-Query
is not sent before the effective leave takes place.
Multiple User Mode allows several users on the same port/VLAN. If one user leaves the group and
other receivers exist for the same stream, the stream continues. The switch achieves this by
tracking the number of receivers that join a given group. For Multiple User Mode to operate properly,
do not suppress reports. This ensures that the switch properly tracks the correct number of
receivers on an interface.
The Fast Leave feature is particularly useful in IGMP-based TV distribution where only one receiver
of a TV channel is connected to a port. In the event that a viewer changes channels quickly,
considerable bandwidth savings are obtained if Fast Leave is used.
You can implement Fast Leave on a VLAN and port combination; a port that belongs to two different
VLANs can have Fast Leave enabled on one VLAN (but not on the other). Thus, with the Fast Leave
feature enabled, you can connect several devices on different VLANs to the same port. This
strategy does not impact the traffic when one device leaves a group to which another device is
subscribed. For example, you can use this feature when two TVs are connected to a port through
two set-top boxes, even if you use the Single User Mode.

Last Member Query Interval tuning

When an IGMPv2 host leaves a group, it notifies the router by using a Leave message. Because of
the IGMPv2 report suppression mechanism, the router is unaware of other hosts that require the
stream. Thus, the router broadcasts a group-specific query message with a maximum response time
equal to the Last Member Query Interval (LMQI).
Because this timer affects the latency between the time that the last member leaves and when the
stream actually stops, you must properly tune this parameter. This timer can especially affect TV
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 is also dependent
on the robustness value, so a value of two equates to a leave latency of twice the LMQI.
Determine the proper LMQI setting for your particular network through testing. If a very large
number of users are connected to a port, assigning a value of three may lead to a storm of report
messages when 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 ten may lead to frequent congestion
in the core network.
June 2016 Planning and Engineering — Network Design
Comments on this document? infodev@avaya.com
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