3750 stack question (backup config when stack is broken)

[upsilon22222]

Hey all,

Curious question.  Let's say I have two 3750's stacked in a ring fashion and the stack cables are pulled thus breaking the stack.  How can I ensure that my switches don't mirror each others config? IP addresses set to interfaces or SVI's may be the same in both thus creating confusion.  I would imagine there is some sort of failover or solution for this type of scenario (should it ever occur)?

[Bilal Nawaz]

Hello, The stacks would then become their respective 'masters' albeit that config is on both. Remember they are referenced to switch numbers.

i.e.

Switch 1 would have all the interfaces gi1/0/1

Switch 2 would have all the interfaces gi2/0/1

Both switches keep a record of the same config if they are stacked.

They are active in aspects of the configuration that is relevant to the switch. If it knows that it is switch 1, it will act upon config for that. With regards to SVI's and the rest of it, its a really interesting question. I am not too sure to be honest. I assume they will try to keep the SVI's active?

Am very interested to know.

hth

Please rate useful posts and remember to mark any solved questions as answered. Thank you.

[Leo Laohoo · Accepted solution]

How can I ensure that my switches don't mirror each others config?

Unfortunately, they do.  That's how stack redundancy.  Each member of the stack takes an independent copy of the entire stack (including themselves).  This is to ensure that in any event, if any switch member of the stack goes off, then the rest of the stack operates normally.

This is one reason why it's recommended that the stack members need to be connected using TWO stack cables instead of one.  IF you do you happen to connect two properly, then there's really a minute risk of a "split brain" occuring somewhere in your network. 

[upsilon22222]

Hey thanks you two, this helps clear things up! I guess it's very uncommon for stack cables to fail?

[paolo bevilacqua]

Dustin Hammond wrote:
 Hey thanks you two, this helps clear things up! I guess it's very uncommon for stack cables to fail?

It's not just uncommon, it's impossible, unless deliberate action is taken.

[Leo Laohoo]

I guess it's very uncommon for stack cables to fail?     

Thanks for the ratings.

I agree with Paolo.  It's very, very rare or unlikely for the cables to fail.  If, however, you're dealing with legacy 3750/3750G, the most likely culprit is a production defect of the stacking module.  The cables will never fail, particularly when it's seated correctly and the s_crews are finger-tight.