Allied Telesis AR400 manual When you enable fast leave on switch

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.

Client 2

1

Switch 3

1 Client sends Leave message

Switch 3	2	Switch 1	Switch 3 forwards Leave message
		2
Switch 3	3	Switch 1	and deletes snooping entry
			Switch 1 sends Membership Query
		3

with 2 second timeout period

igmp-fast-leave.eps

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.

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

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

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.

Page 65 AlliedWare™ OS How To Note: IGMP