Allied Telesis AR400 manual Potential problems with changing these counters

Configurable IGMP timers and counters > Last Member Query Count and Last Member Query Interval

Potential problems with changing these counters

For most networks, the default LMQI and LMQC values work. You should only change them if you are aware of the likely effect on the network. In particular, note that:

zChanging the LMQC automatically changes the Robustness Variable. Therefore, we do not recommend setting the LMQC to 1, because it removes the system’s allowance for packet loss. See page 75 for more information about the consequences of changing the Robustness Variable.

zIf you set the LMQI (or LMQC, or both) too low, clients will not be able to reply to Specific Queries quickly enough and the Querier and Snoopers may delete group entries for ports that still need to receive multicasts. If this happens, some or all clients briefly lose the multicast stream.

The default values of LMQI (10) and LMQC (2) mean that the Querier must receive client Membership Reports within 2 seconds of the first Query. This is already quite a short time and we do not recommend reducing it even more. For example, reducing the LMQI to 5 would allow only 1 second for responses, which may be too little.

How to change these counters

The following example increases LMQI a lot, and then shows the resulting changed refresh time.

Manager Switch 1> set ip igmp lmqi=255

Info (1005003): Operation successful.

Manager Switch 1> show ip igmp

IGMP Protocol

------------------------------------------------------------------------

Status	Enabled
Default Query Interval	125	secs
Default Timeout Interval	260	secs
Last Member Query Interval	255	(1/10secs)
Last Member Query Count	2
Robustness Variable	2
Query Response Interval	100	(1/10secs)
Disabled All-groups ports	None

Interface Name	vlan100	(DR)
Group List
Group. 224.12.13.14	Last Adv. 172.31.0.222		Refresh time 49 secs
Ports 49

------------------------------------------------------------------------

The refresh time in seconds is (LMQI/10) * LMQC = 255/10 * 2 = 51 seconds

So with the new LMQI setting of 255 and the default LMQC setting of 2, the IGMP Querier waits 51 seconds for a Membership Report to arrive before it ages out the IGMP entry. Note that the output above displays 49 seconds, because it took us 2 seconds to enter the command show ip igmp.

Page 73 AlliedWare™ OS How To Note: IGMP