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Upgrading with ISSU

posted in Cisco Networking
by on May 14th, 2012 tags: , , ,

I recently had to upgrade the IOS on a Cisco 4500.  I figured it was the perfect opportunity to try out ISSU and to blog about it.

ISSU (In-Service Software Upgrade) is a feature that allows the operator to upgrade (or downgrade) the IOS on device without having to take the entire device down and potentially impact service. This is partly accomplished with the SSO supervisor redundancy mode that performs the subsecond supervisor switchovers while each supervisor is restarted and loaded with the new (or old) IOS.

In brief, the process works as follows: Both supervisors are running the old image. The standby supervisor is restarted and loaded with the new image. The operator switches over to the standby supervisor to ‘test drive’ the new image. The switchover automatically restarts the active supervisor, bringing it up as the standby and making the current standby supervisor the active.  Finally, the standby supervisor, which started as the active, is restarted and loaded with the new image. In the end, both supervisors have the new image.

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