I have a pair of 7301 routers in Active/Standby mode using HSRP and they also have the radius sticky database synchronization enabled. Their basic job is Load Balancing HTTP and radius traffic with IP SLB.
Currently, I use 12.2.18 IOS and I want to go to 12.4.X IOS.
I am thinking of 2 possible ways for the upgrade.
One way is to upgrade the routers one by one. So, the first will be reloaded to the new IOS while the other is still on old IOS.
After a short period the second router will be upgraded as well.
In this case there will be a short time period where both routers are in a HSRP configuration but using different version of IOS.
How are they going to behave? I don't know if the HSRP is going to complain about the IOS version difference and if the radius sticky database will kept synchronized.
Another way is to upload the IOS on both routers and reload them at the same time, or with few secs delay.
Which of the above methods is the safest?