Allied Telesis AR400 manual How clients leave groups queries and timers, Overview of leave process

How clients leave groups: queries and timers > Overview of leave process

How clients leave groups: queries and timers

When a client leaves a group, the Snoopers and the Querier check which ports now have clients that belong to that group. They will stop forwarding the group’s traffic out any ports that are now unnecessary. In this section, we describe the process in detail.

Overview of leave process

The basic process when a client leaves a group is as follows:

1.The client sends a Leave message to indicate that it no longer needs to receive that multicast group.

2.The Snooper receives the Leave message and forwards it towards the Querier.

3.For all ports that belong to the group, the Querier changes its internal group membership timer to a short value (2 seconds by default—see Querier timer values below).

4.The Querier sends a Specific Query to ask which other clients still belong to that group.

5.The aforementioned Snooper receives the Specific Query. For all ports that belong to the group, the Snooper changes its internal group membership timer to a short value

(2 seconds by default—see Snooper timer values below) unless the timer is already short. It forwards the Query out all its ports.

6.The Querier waits for the Last Member Query Interval time, 1 second by default, and then sends a second Specific Query.

7.The aforementioned Snooper snoops this second Specific Query and uses it to set the internal group membership timer for each port, unless the timer is already short (which it will be if the Snooper received the first Query). It forwards the Query out all its ports.

8.If the Snooper or Querier receives a Membership Report on a port, it sets the port timer to the Default Timeout Interval value and continues to forward the multicast stream out that port. Otherwise, the timers for that port expire and the Snooper and/or Querier stops forwarding the multicast stream out that port.

Querier timer values

As described in Step 3 above, when the Querier sends a Specific Query for a group in response to a Leave message, the Querier updates a timer for ports that forward that group. The timer is the following two values multiplied together:

zLast Member Query Count (LMQC)—the number of Specific Queries the Querier sends, 2 by default, and

zLast Member Query Interval (LMQI)—the time between the Specific Queries, 1 second by default

The default LMQC and LMQI give a timeout of 2 seconds. Therefore, by default the Querier must see the client response within 2 seconds of sending the first Specific Query.

Because of this process, sensible values for LMQC and LMQI are essential. In most networks, the defaults are appropriate and you should not change them. If you need to change them, see "Last Member Query Count and Last Member Query Interval" on page 72.

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