Cisco 3750 Stacking Question

[nmjones]

Hi

Can anyone advise me if in a stack of 3750's, if the master was to fail and another switch in the stack had to take over as Master. Is the transition "totally" seemless with no break in service at all (other than anything connected to the failed switch of course) ?

The reason why I ask is we currently use stacked switches from another vendor and recently the Master switch failed and there was a disruption of service across the stack for around 30 seconds whilst the new master did some kind of soft reset to load the master config to run the stack again. This wasn't acceptable as clustered devices got upset. So I am wondering if in this case Cisco is a better option.

Many thanks in advance

Nick

[Reza Sharifi]

Hi Nick,

Here are some numbers provided by Cisco.  It depends on if you are running the stack as layer-2 or 3.

Reconvergence times tested under heavy load:

Layer 1 failure is detected in several microseconds

Layer 2 failure ~ mseconds

Layer 3 link failure—sub 200 mseconds

Layer 3 member failure—sub 300 mseconds

Layer 3 master failure—up to eight seconds

HTH

[Joseph W. Doherty]

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Posting

Besides what Reza has provided, when a 3750 stack master fails, by default, gateway MAC, if hosted on the stack, will change.  This might "confuse" hosts that don't process the gractious ARP.  Two ways to mitigate this, there's an option to retain the original master's MAC or use HSRP.

If the stack is doing dynamic routing, its peer/neighbors might see the interruption too.  If NSF is enabled, this interruption can be minimized.

[nmjones]

Thank you both for the answers, they are both excellent and give me plenty to offer back to the business.

I guess ultimately the best option is a Chassis based solution though as there is still a potential for disruption.

Many thanks

Nick