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There's this Cisco Catalyst 6509 switch with 2 (two) supervisor modules; one of them is active, the other is (almost always) in standby.

We need to upgrade IOS; current version is 12.2.something (I can check exactly which one, if this does matter).

Can this be done without service interruption? Something like "upgrade the standby supervisor, activate it, upgrade the other one"?

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up vote 2 down vote accepted

You want to do a Enhanced Fast Software Upgrade (eFSU). It is supported in versions 12.2(33)SXI and later.

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The IOS version is earlier :-( – Massimo Mar 16 '10 at 19:24

According to Cisco, it is possible to upgrade a 6500 running IOS in native mode without powering down the switch.

However, I have never seen this done in production. The procedure I have seen followed most often is that a supervisor is upgraded off line, and the config pasted in via console. The switch is powered down, both old supervisors are removed, and the switch is booted with the new supervisor. Once it is confirmed to be up and running clean, a second supervisor running the new code version is inserted.

In the document above, Cisco also mention:

Note: This procedure can impact the data traffic. 
Cisco recommends that you perform this procedure 
during a scheduled maintenance window.

I would suggest that you get an agreed maintenance window, and advise your users that there might be service interruption. This is usually much preferred to advertising no outage, and subsequently causing one. :)

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Are you talking about using a third supervisor module to do the upgrade, and physically exchange them?!? This really puzzles me... why don't you just load the new IOS on the existing ones and reboot the switch?!? – Massimo Mar 16 '10 at 19:23
Having the old supervisors available means that rollback is easier. As some code upgrades change the format of configuration, rolling back by changing the 'boot' statement is not always guaranteed to restore the original configuration. – Murali Suriar Mar 16 '10 at 19:56
Obviously a backup is in order prior to ANY O.S. upgrade :-) I don't think many people have an additional supervisor module available just for using it when doing IOS upgrades... – Massimo Mar 16 '10 at 20:36
True - if you don't have any spares at all, then this isn't a viable option. That said, unless you are paying for 4 hour response on your smartnet, you could be waiting a long time for replacements should you have a failure. :) – Murali Suriar Mar 16 '10 at 21:28

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