Traffic Behavior with failed VSL Links in VSS Configuration

[Terence Lockette]

Hello,

I have a question regarding VSS I thought I'd post here.  According to Cisco, "If for some rare reason all VSL connections are lost between the virtual switch members leaving both the virtual switch members up, the VSS will transition to the dual active recovery mode (http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-6500-virtual-switching-system-1440/prod_qas0900aecd806ed74b.html)."  How does this affect traffic for switches connected via MEC?  Will it began using STP/MST, if configured, for switches with dual links to different chassis?  Just curious.  I plan on testing various scenarios prior to putting these chassis into production by simulating some of the common network outages we've had in the past.  Thanks in advance!

Regards,

Terence Lockette

[Madhukrishnan Gopinathan Nair]

Hi Terence,

For devices attached to the VSS it is seen most likely as a member port down in Port-channel.

An VSS active chassis that detects a dual-active condition shuts down all of its non-VSL interfaces (except interfaces configured to be excluded from shutdown) to remove itself from the network, and waits in recovery mode until the VSL links have recovered

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/vss.html#wp1258876

Hope this helps. Please do remeber to rate all helpful posts

Thanks,

Madhu.

[Terence Lockette]

Madhu,

So does this mean that any host connected to the a chassis that waits in recovery mode will lose connectivity to the network?  Would I be able to administratively bring up these interfaces in the event that this happens?

[Madhukrishnan Gopinathan Nair]

Yes if it only single legged. But the whole purpose of VSS is you can have MEC (Multichassis ether channel) and if you have only one connection to the chassis, it is against the concept of high availability.

I dont think you can bring up any interfaces, but you may have an option to exclude interfaces not to be suspended.  You can refer the above link for same.

Also some useful Q& A on VSS can be found below

http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-6500-virtual-switching-system-1440/prod_qas0900aecd806ed74b.html

Hope this helps. Please always remember rate useful posts.

Thanks,

Madhu.

[Terence Lockette]

I guess I should clarify my concern.  All servers would be connected to a ToR switch such as the 3750-X.  These ToR switches will have an MEC configuration so if one chassis in the VSS fails, connectivity continues on the unbroken link.  However, we would also have other devices such as routers and our firewall that would connect directly to one of the VSS chassis.  If, for example, all VSL links fail, a router or firewall connected to one the active mode chassis would lose connectivity if the switch shuts down all interfaces.  Would this be recoverable, assuming we aren't able to get a single VSL link back up to restore the VSS?

Regards,

TLock