HSRP – Decrementing Above Assigned Priority

HSRP is something that allows for redundant gateways to be provisioned on the network.

It can be assigned to an interface or an SVI.

A priority needs to be assigned to HSRP. If unassigned, the default priority assumed is 100.

Interface tracking can be configured with HSRP so HSRP priority can be decremented if the interface goes down. Decrementing is useless without preemption though. Configuring preemption means the standby HSRP interface can take over if it has a higher priority.

But what happens if you decrement HSRP by higher than its value?

For example, you perform the following configuration on two HSRP interfaces:

Switch 1

interface vlan 100
ip address 100.1.1.3 255.255.255.0
standby 100 100.1.1.1
standby 100 priority 150
standby 100 preempt
standby 100 track 100 decrement 50 //this is dependent on tracking object 100 being configured

Switch 2

interface vlan 100
ip address 100.1.1.2 255.255.255.0
standby 100 100.1.1.1
standby 100 priority 120
standby 100 preempt
standby 100 track 100 decrement 50 //this is dependent on tracking object 100 being configured

Assume HSRP timers are at their default values of 1 and 3 seconds.

If VLAN 100 goes down on Switch 1, then Switch 2 will become active in 3 seconds.

However, what if Switch 1 was configured with a decrement priority of over 150? Say 200?

A senior engineer told me that this would result in the Standby HSRP group immediately taking over. So I tested it.

He was wrong. It still took 3 seconds for it to take over. The priority decremented to 0 and not to a negative amount. I’ll put up the show command output once I get a chance. This was previously tested on a test lab using Catalyst 4948 switches. I will try and replicate it using 2600s and 2950s.

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