VSS Recovery after VSL Failure

Unanswered Question
Mar 21st, 2010

Hi,

I am using VSS configuration on a pair of 6509s with IOS 12.2(33)SXI3. After VSL link goes down the standby chassis sucessfully detects the complete VSL failure and initiates SSO correctly. This causes a dual-active scenario and the former active chassis shuts down its ports (recovery action). During the time VSL is still not operational if the current active (previous standby) chassis fails this is not detected by the chassis in recovery mode and this results a complete failure of both of the chassis. My question is: Is this an expected behaviour and how (if possible) can I configure the chassis in recovery mode to reload automatically and assume the standby role so at least one chassis from VSS is functional in case of power outage for example?

My suggestion is to exclude the MEC port channel interfaces (connected to access layer switches and used by Enhanced PAgP) from the automatic shutdown in case of recovery scenario, but I am afraid this could cause some other undesired effect (like spanning tree loop or some other L2 problem). I am not sure if this will help, but I expect VSS chassis in recovery mode to detect the active chassis failure and take some action to become active in turn.

Additional info: If the VSL link is operational again, the chassis in recovery mode detects that and reloads automatically. Also, I am not making any config changes on the chassis in recovery mode.

Please advice if this is possible or recommended but at least I need to know if this is an expected behavior. I could not find this scenario in the configuration guide I used when configuring VSS: http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/vss.html#wp1083292.

If you need any configuration details, please let me know. Thanks in advance!

Regards, Kliment

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Giuseppe Larosa Mon, 03/22/2010 - 01:44

Hello Kliment,

I would implement dual active detection with IP BFD and/or fast hellos

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/vss.html#wp1063718

to be noted:

you don't need to use a tengiga link for this but you can use GE ports as explained in configuration example or even an FE port

these methods use a direct routed link (one or more) between member chassis of VSS.

The use of multiple methods of active detection is recommended as a safety measure.

with MEC and enhanced PAGP you need to use a third party device to perform dual active detection.

Hope to help

Giuseppe

kimby200602 Mon, 03/22/2010 - 02:08

Hi Giuseppe,

Thank you for the quick answer

Definetely BFD will help me with dual-active detection, but I think this detection is working fine now with Enhanced PAgP. As I have mentioned previously, the former active chassis is correctly entering recovery mode, but it is not auto reloaded to take the standby role if the VSL is not up again.

Do you think that BFD will solve this scenario or it is a known issue?

Thank you again!

Regards, Kliment

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