HSRP Documentation Question

[JohnTylerPearce]

I have a question about HSRP documentation. In several instances, with just two routers on a broadcast network, I see preempt on both routers. This doesn't make much sense to me, since I only need it on the Active in this scenario

R1

----

int fa0/1

ip address 192.168.1.2 255.255.255.0

standby 1 ip 192.168.1.1

standby 1 preempt

R2

----

int fa0/2

ip address 192.168.1.3 255.255.255.0

standby 1 ip 192.168.1.1

standby 1 priority 91

With the following configuration, if I shut down the fa0/1 port on R1, R2 will become th active router (This is to be expected).

If I then, do a no shut on fa0/1, R1 becomes the Active router due to the preempt.

In similar HSRP documentation (Actually a lot) I see preempt on both. Is there a reason for this? Was there a time where preempt was needed on the standby?

[Peter Paluch · Accepted solution]

Hi John,

In a scenario with just two routers and no object tracking, just the routers backing each other up, there is indeed no need for preempt on the Standby router. However, partly because of more difficult understanding of "who can preempt whom and when" and partly because object tracking absolutely requires preemption as routers do not go away but merely adjust their priorities, it is routinely recommended to activate preemption on all routers which makes the behavior deterministic and hence predictable. Adding the object tracking later is greatly simplified by this.

Best regards,

Peter

[JohnTylerPearce]

Thanks Peter, I appreciate the response. I think a lot of documentation, belives that no matter what, you need preempt which is not true.

[Peter Paluch]

Hi John,

This may be true that within the context of the documentation, putting the preemption everywhere may not make sense. Then again, I am one of those people who actually advocate for the preemption to be summarily activated on all HSRP speakers. Usually, the preemption is what you want. Solving problems with the planned active router not actually seizing its active role back and trying to wrap one's head around peculiar combination of preempt/nonpreempt configurations is not particularly rewarding. VRRP has preemption on by default. So I am personally not annoyed nor surprised to see the HSRP preemption activated with no apparent reason.

Best regards,

Peter

[JohnTylerPearce]

I guess if you really think about it, having preempt on R2, in the above configuration example, really isn't going to hurt anything. And if you were to put the following configuration on R1, then preempt would be needed on R2, lilke you mentioned

R1

----

int fa0/1

standby 1 track s1/0 15

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

John, as already described by Peter and you, for correct functioning with the existing settings, preempt wouldn't be actually needed on the secondary.  I've often seen it configured in production environments just to make a more consistent configuration across multiple devices and for flexibility in using HSRP, i.e. you don't have to "remember" to add or remove preempt if you decide you want to reset priorities.  For example, primary path is having some network performance issues, but interface is up.  So you want to temporarily make the existing secondary the primary.  So, you only need to adjust one of the two gateways priority.  When you have 3 gateways or more, do you really want to keep in mind, only the gateway with the lowest priority doesn't actually need the preempt statement?

[Peter Paluch]

Hi Joe,

Thanks for joining Yes, absolutely agree.

Best regards,

Peter