Allied Telesis AR400 - Manual (Page 65)

Page 65 | AlliedWare™ OS How To Note: IGMP
IGMP fast leave > Explanation of IGMP fast leave
When you enable fast leave on switch 3
When Fast Leave is enabled on switch 3, but not on switch 1, an interesting chain of events
occurs when the client sends a Leave message, as shown in the following diagram.
The result of this is that switch 3 adds the group back into its snooping table (with the same
timeout as the IGMP Querier) but has no ports interested in receiving the group. Because
Fast Leave is enabled on port 3, that port was removed from the group as soon as switch 3
received the Leave message.
Adding Fast Leave to switch 1 would not be sensible, since there may be clients attached to
other ports on switch 3. If you enabled Fast Leave on switch 1, one Leave message from
switch 3 would drop the multicast stream for everyone on that switch.
It is safe to ignore the group entry on switch 3.
Manager Switch 3> show igmpsnooping
IGMP Snooping
------------------------------------------------------------------------
Status ........................... Enabled
Disabled All-groups ports ........ None
Vlan Name (vlan id) ..... default (1)
Fast Leave .............. On
Group List ..............
No group memberships.
Vlan Name (vlan id) ..... vlan100 (100)
Fast Leave .............. On
Group List ..............
Group. 224.12.13.14 Entry timeout 2 secs
Ports None
All Groups Entry timeout 258 secs
Ports 26
------------------------------------------------------------------------
Client 2
Switch 3 1
Switch 1 Switch 3
2
Switch 1 Switch 3
3
1 Client sends Leave message
Switch 3 forwards Leave message
and deletes snooping entry 2
Switch 1 sends Membership Query
with 2 second timeout period 3
igmp-fast-leave.eps