Running VSS with a single VSL

[Jeremy Grant]

Morning Everyone,

I am trying to convince our Technical Lead that dual VSL links between our 6509-E chassis is a VERY good idea but he is fully convinced that the setup will work perfectly fine with only one VSL link.   As I do agree that we can do this, I am looking for more concrete evidence that shows this would be a bad setup.

I know that we will lose the High Availably that comes with VSS, what I don't know is what will happen to all of the downstream switches that are connected and what will happen to the two 6509-E chassis..

Can someone please share their thoughts on this?

Thank you.

[Reza Sharifi]

Hi,

al though it works with one link only, it is recommended and a good idea to have redundant VSL links. The reason for that is if you only have one link and that one link fails, the switches go to dual active mode and the primary switch will shut all its port and let the backup switch take over, therefore you only have one device forwarding.

Here is more info:

If the VSL fails, the VSS standby chassis cannot determine the state of  the VSS active chassis. To ensure that switchover occurs without delay,  the VSS standby chassis assumes the VSS active chassis has failed and  initiates switchover to take over the VSS active role.

If the original VSS active chassis is still operational, both chassis are now VSS active. This situation is called a dual-active scenario.  A dual-active scenario can have adverse affects on network stability,  because both chassis use the same IP addresses, SSH keys, and STP bridge  ID. The VSS must detect a dual-active scenario and take recovery  action.

Link:

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

HTH

[Jeremy Grant]

Thank you very much, that was exactly what I was looking for.

[Jeremy Grant]

One last question, what would be the recommended bandwidth of the ports be?  We have two ten gig ports on the sup cards but one of them has already been committed for another role.  Does the VSL have to be 10gig or can it just be 2-4 1gig ports?

[Reza Sharifi]

VSL links HAVE to be 10Gig.

Use one port from the sup and one port from a line card to create your VSL links.

HTH

[Leo Laohoo]

Don't forget to factor in the VSS heartbeat link.