Cisco Support Community
Community Member

CGMP leave vs CGMP fast leave

Hi all,

With CGMP you can configure CGMP fast leave:

CGMP fast leave enables the layer 2 switch to snoop IGMP v2 leave packets and directly send an IGMP membership query on the port from where it received the IGMP v2 leave to check if there are still hosts interested in the multicast group on the port.

Then, the MAC address of the user is removed from the multicast layer 2 forwarding table of the access switch.

If there are no more hosts on the access switch, the leave message is sent on the uplink to prune the access switch completely (same process).

My question is: if I don't use CGMP fast leave but cgmp only, will my IGMP leave message trigger a CGMP leave from the querier even if the host leaving the group is not the last one in the multicast group?

According to Cisco documentation, it won't be the case. But on one of my testbed, I saw CGMP leave messages triggered by the querier even when it was not the last host leaving the group. So was it a bug or a normal behaviour?

I would rather have these CGMP messages triggered all the time since they would prevent flooding on non-interested ports.

Thanks for your help,


Cisco Employee

Re: CGMP leave vs CGMP fast leave

If you have only cgmp enabled and see a CGMP leave message that would

indicate that the CGMP server (the router) thinks there are no more receivers

in that group and thus sends a CGMP leave for that group.

So you might wanna run debug ip cgmp and debug ip igmp on that router

and see what might be going on

Community Member

Re: CGMP leave vs CGMP fast leave


So since my multicast flows will be quite heavy and I need to optimise the load on all clients, I'll need to configure CGMP fast leave.

I'm a bit confused with the HSRP address conflict.

On a simple 2 layer VLAN (2 redundant cores and access switches directly dual connected on themIf I enable CGMP fast leave on access switch only (not on cores), HSRP packets will not be snooped and I shouldn't introduce any latency in HSRP process. Am I right?

Do I need to enable fast leave on all switches on my VLAN?

Is anybody using fast leave in a production environment? Any feedback?



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