We are running several 4500-X VSS setups and we would like to upgrade them from 3.4.x to 3.6.x. Cisco states that ISSU is not supported due to incompatible software changes. So I asked TAC if it was possible to do an upgrade while keeping only layer 2 traffic active. TAC suggested the following upgrade procedure:
- Copy the image on both the switches [bootflash: <image name> - Configure the config-reg 0x2102 - Reload the stanby switch and it will boot from new image [3.6.4] - Check the redundancy state, standby will show Standby cold - Then, you can do a force switch over
Has anyone tried this? Will this keep layer 2 active during the upgrade?
TAC also claims that multichassis etherchannels *should* remain active. Can anyone verify this?
There will be no problem. What TAC is suggesting is that, as long as there is an active chassis in VSS there should be no forwarding issues as long as it does not get congested since the BW available will drop to 50% meanwhile the standby chassis stays in standby cold.
About the MEC, as I said, only the links member of port-channels that stay active in the active supervisor will forward traffic but It will not be an outage.
Please do not forget to point the boot variable to the new IOS image.
What I'm still worried about a bit is what happens to the LACP / MEC during the switchover from the active 3.4 to the standby cold 3.6. I assume that the attached servers need to renegotiate the LACP protocol which can cause a brief downtime. Is that correct?
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