HSRP preempt question

[pcclonescisco]

Our setup has two routers (Core-1 and Core-2) running in HSRP active standby mode and multiple customers are segregated with vlans. we have enabled preempt command on both the core routers.

Up to my understanding,

> Preempt command should be configured in both the routers when tracking is enabled and also configuration should be done in active( primary) router to decrement the priority value such that priority of standby(secondary) router is made high and standby router becomes active,when the active router's up-link interface's line protocol goes down(We are tracking the line protocol of up-link interface)

> When tracking is not in place, preempt is enough to be configured only in the primary router.

Is this correct?. Also, what will be the effect when preempt command is configured in both the routers when we are not tracking the up-link interface.

[viswamin · Accepted solution]

Preempt may be required on the standby router if you want it to become active based on the priority assigned and not on the order of boot.

assume a case where is R1 is configured with priority of 95 and R2 will a priority of 100 and you have configured preempt only on active.

when the routers boots up in this order R1 followed by R2, R1 will assume the role of active and R2 will be standby since preepmt is not configured although it has the higher priority.

if R1 goes down, R2 will become active and when R1 comes back again, then preempt would not have any affect as the priority is compared. so it matters only during the initial bootup .

Just my 2 cents.

-Vijay

[Latchum Naidu]

Hi,

The HSRP preemption feature enables the router with highest priority to immediately become the Active router. Priority is determined first by the priority value that you configure, and then by the IP address. In each case a higher value is of greater priority.

It is my understanding that preempt works when there is a priority difference and allows the higher priority router to take over. When the priorities are equal I am not convinced that preempt works based on address relationship. Devang is convinced that preempt does work based on address relationship.

Please rate all the helpfull posts.
Regards,
Naidu.

[JohnTylerPearce]

Well also, let's say you have R1 and R2. R1 has a priority of 100 and R2 has a priority of 95. In this example, R1 and R2 both have preempt configured. Let's say R1 goes down, and which makes R2 the active router on the HSRP standby group. If R1 comes back up, it will take over as the active router since it does have higher priority and preempt configured.

If you take the same scenario, except for R2 only has preempt configured, if R1 goes down, R2 will become active, but if R1 comes back up, even though it has the higher priority, it will not become active, since it does not have preempt configured. In theory, for the basic principle of HSRP to work you don't really need preempt but I highly recommend it

Of course you can also track interfaces and status as well. I would highly recommend that.

[JohnTylerPearce]

Naidu I think when both priorities are equal, it's based on highest IP address, I could be wrong though.

[Latchum Naidu]

John,

You are not wrong
The priority field is used to elect the active router and the standby router for the specific group. In the case of an equal priority, the router with the highest IP address for the respective group is elected as active. Furthermore, if there are more than two routers in the group, the second highest IP address determines the standby router and the other router/routers are in the listen state.

Please rate all the helpfull posts.
Regards,
Naidu.