Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
Community Member

Fast SSID Change - Mac Flapping

Has anyone seen mac flapping being reported on upstream switch from controller when Fast SSID is enabled?  This is as a reult of a client that is going back and forth between one WLAN and another.

My understanding is that even with this value enabled, the controller would remove that client/mac from the first WLAN/VLAN prior to moving it to another WLAN/VLAN but would that immediately clear the cam tables on the upstream switch?  Is it just happening so fast/often that it's interpreted as a mac flap?

I've read many discussion about enabling this to allow clients to make that choice but have not found anything negative about it.  It's interesting that the value by default is disabled and I've read that the delay is roughly 20 seconds.  Why would this be? 

Thanks in advance!  //art

Everyone's tags (5)

Re: Fast SSID Change - Mac Flapping


CAM table on switches is not updated by the client's SSID change. This is why you get the MAC flap.

I get the mac flap on different scenario when a mesh AP changes the parent and it s mac will appear on two different access switches at the same time so the distribution switch reports a mac flap message.



Sent from Cisco Technical Support iPad App

Rating useful replies is more useful than saying "Thank you"
Community Member

Re: Fast SSID Change - Mac Flapping

Thanks Admjad - I understand that there is no direct relation to a client WLAN change triggering a switch cam table update.

Rather, I'm wondering more about the definition of a mac flap on a switch.  If a switch sees a mac address on one port and then sees it on another, it would simply update the cam table with the new port, reset the age and move on.  What I'm wondering, is if there is a specific trigger condition for a mac flap - meaning, this condition would need to happen so many times during a set amount of time before saying it's a mac flap.  AND if that is the case then we should only really be seeing mac flaps (with regards to wireless) when we have a client that is sitting between two ap's that are on different controllers and the client is jumping back and forth between the two. 

I was also wondering if the ~20 delay in switching WLAN's was introduced (by turning off Fast SSID) to prevent the conditions for a mac flap from being met and then not triggering a mac flap notification by the switch.

Thanks in advance //art

Re: Fast SSID Change - Mac Flapping

Art: i think (not sure) that it is considered a fla if the mac source changes without the interface go down. Updating the table is done but the update contradicts with another entry that s on the table an hence a host-flap is triggered. For 1 host-flap msg that would be normal in our situation but you should care bout it if there are too many host-flap messages generated for same ac.

Rating useful replies is more useful than saying "Thank you"
CreatePlease to create content