Altering Igmp Query Intervals - HP 7102dl - ProCurve Secure Router Configuration Manual

Configuring Multicast Support for a Stub Network
Configuring IGMP Proxy for Multicast Stub Routing Support

Altering IGMP Query Intervals

IGMP involves trade-offs. The protocol contains packets by giving multicast
routers up-to-date information on which networks actually need specific
multicasts. On the other hand, the IGMP queries that maintain this information
also consume bandwidth.
Although the default settings are usually adequate, you can alter IGMP inter-
vals. For example, in a network with relatively stable group memberships, you
may determine that routers are sending too many IGMP messages. In that case,
you could raise the IGMP query interval. Or, you may determine that hosts are
not receiving multicasts quickly enough after joining a group. In this case, you
could lower the IGMP query interval so that the router receives an updated
IGMP report more quickly.
You can also reduce overhead by enabling the immediate leave option when
an interface connects to a single host. When this host leaves a group, the router
assumes the group has expired without sending any query messages. You can
also enable this option when the router connects to a switch that uses IGMP
snooping. Such a switch forwards all multicasts until it specifically receives a
leaves from all connected hosts. Therefore, if an IGMP snooping switch does
sends a leave for a group, the router does not need to check whether other
hosts still need the multicasts.
Table 12-1 explains how to alter IGMP intervals. You enter these commands
from the logical interface configuration context.
C a u t i o n
You should not alter intervals unless you have experience working with IGMP.
Whenever you adjust these intervals, you risk making a router's list of group
memberships and multicast routes less accurate. The timers on interfaces in
the same subnet need to be consistent. For example, if you raise one interface's
query interval, you need to ensure that other interfaces running IGMP have a
correspondingly greater query timeout. Otherwise, they will periodically flood
the network with unnecessary messages, believing that the designated multi-
cast agent has gone down.
12-19