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Page 73 | AlliedWare™ OS How To Note: IGMP
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:
z Changing 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.
z If 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.
The refresh time in seconds is (LMQI/
1
0) * LMQC = 255/
1
0 * 2 = 5
1
seconds
So with the new LMQI setting of 255 and the default LMQC setting of 2, the IGMP Querier
waits 5
1
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.
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
------------------------------------------------------------------------