The standby preempt command enables the Hot Standby Router Protocol (HSRP) router with the highest priority to immediately become the active router. Priority is determined first by the configured priority value, and then by the IP address. In each case, a higher value is of greater priority. When a higher priority router preempts a lower priority router, the router sends a Coup message. When a lower priority active router receives a Coup message or a Hello message from an active, higher priority router, the router changes to the Speak state and sends a resign message.
The preempt command allows the device with the better priority to take over the role of Active, without having to wait for the current Active device to go down.
So, in your example, i expect that when you configured the first device, it became the active router for both groups (as there was no backup present) then, when you configured the backup without the preempt command, it would not have taken over as active until the first device failed.
As a rule of thumb, especially if you are using interface tracking, always ensure preempt is enabled.
However, in the 1st step i didn't configure the standby preemt, assuming that the router with the highest priority would become the active router.
However, i noticed that the router with the highest IP was the one becoming active, leading to have it active on both the groups (this is what i wasn't able to explain). Although i rebooted it twice, i had no changes until i configured the standby preemt on both the routers
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