We have two core N9K switches running HSRP on them. We have an upcoming project that is requiring us to upgrade the software in order to support for HSRP instances. This is bringing up a unique scenario that I couldn't find too many resources on.
Do the two Nexus 9ks that are performing the HSRP routing need to be running the same version software? Eventually they will be, but during the upgrade process we will be brining one down and waiting until it comes back up with the new OS then upgrading the second one in order to help minimize downtime. Does anyone know if that small window where they both will be on, but running different OS versions, will cause any issues?
It is vPC and I had done a small amount of reading on that but wasn't sure if we were going to run with that or just simply do a standard upgrade and run with the reboot of each switch individually. Possibly using ISSU would also eliminate any potential issues with unmatching OS version during the upgrade and any pitfalls that may come with that?
Currently the version NX-OS we are running only supports 490 SVI's with HSRP, so we need to upgrade to a new OS version to get the bump up to 1000.
@balaji.bandi has correctly identified aspects of doing the code upgrade (ISSU or separately). But in reading the original post I think the focus of that post was on something different: "Do the two Nexus 9ks that are performing the HSRP routing need to be running the same version software?" so perhaps the focus was on the HSRP aspect more than on the general aspect of upgrading the switches.
In my experience if you have switches running HSRP and upgrade one switch to a newer version of code it has never created a problem. Cisco is pretty good about providing backwards compatibility between a newer version of code and an older version of code. The newer version of code might support new features in HSRP, but as long as you do not try to implement the new features till both switches are running new code there should not be a problem.