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4500X VSS Behaviour

packet2020
Level 1
Level 1

Hi All,

 

I am trying to understand what the expected behaviour is with VSS in the following scenario. Unfortunately I dont have access to a pair of non production VSS capable switches to test this out.

 

Say you have a VSS comprising of 2 x 4500X switches. If both switches rebooted without the VSL and dual-active-detection links connected, the two switches will come up as active so introducing a split brain scenario.

 

If the VSL link was then connected while both switches were VSS active (without he dual-active-detection link), what is the expected behaviour? Will both switches reboot to select the active and standby role, or will one switch boot as active with the other booting into recovery mode? 

 

7 Replies 7

Leo Laohoo
Hall of Fame
Hall of Fame

@packet2020 wrote:

If the VSL link was then connected while both switches were VSS active (without he dual-active-detection link), what is the expected behaviour? 


Both physical switch will reboot.  This will minimize any chance of split-brain.

Thanks Leo.

 

The reason I ask is that switch 2 for one of our VSS deployments has been off for a few months and the team are looking to get this back online by booting switch 2 first (with all cables disconnected) to make sure that it is ok and then connect the VSL. If they did this then switch 2 will become as active and then like both switches will reboot once the VSL is connected which we dont want.

 

What is the correct procedure to bring switch 2 back online. Should it be powered off first, VSL connected and then booted to join the VSS as standby? When it boots will it sync its config from switch 1 during boot?

Make sure all links, including the link for the heartbeat, are connected before powering up.  

Hi,

As long as that switch (switch 2) does not have any priority set higher than switch 1(default priority), all you have to do is connect the links including the VSL, dual-active-detection link, and turn it on. Once completely booted, it should boot as switch 2, which is the standby chassis. That said, I would do all of these in a maintenance window, as VSS can sometimes have unexpected behavior, and you do not want to be surprised with an outage during production hours.

 

HTH

Thanks Reza.

 

So when switch 2 boots and joins the VSS, should it sync its outdated configuration with switch 1?

One more thing about VSS:  ISSU and FSU/eFSU -- Do not believe in the hype.  Avoid at all cost.  

A reboot of a VSS pair will take, maximum, of about 10 minutes.  When a ISSU or FSU/eFSU fails, it will take some time to get there and power the chassis down.

So when switch 2 boots and joins the VSS, should it sync its outdated configuration with switch 1?

Yes, as long as switch-2 is converted to VSS, and once the VSL is connected, switch-1 will sync the config to switch-2 and override its existing config.

Once switch-2 is up and running, verify VSS is working with "sh redundancy" and also "sh switch?"

 

HTH

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