HSRP preempt

Question

Hi,
I would need some advise to clarify on the following HSRP preempt concept. Please let me know if the following is correct.
1) both active and standby routers NOT configured with HSRP preempt. When active router goes down, standby will NOT take over due to NO HSRP preempt configured.
2) both active and standby routers configured with HSRP preempt. When active router goes down, standby will take over as the active. When the active goes down again, the standby will take over as active. However, there will be a short downtime during the HSRP transition states. How long is the transition state?
3) only standby router configured with HSRP preempt. When active router goes down, standby will take over to become the active. However, when the active router goes down again, the standby router will NOT take over as the active due to NO HSRP preempt configured.
Thanks
Christina

Answer 1

Christina
1) Not correct. If both routers do not include preempt and the active router goes down then the standby router WILL take over as the active. Preempt is not an issue here. If the currently active router goes down and if there is a standby then the standby will take over as active. In a failure situation the standby will take over without consideration of preempt.
2) I am not entirely clear what you are describing when you say the active goes down and then when the active goes down again. So let me re-phrase it in terms of rtrA and rtrB. Assume that rtrA has a higher priority and becomes active and rtrB is standby. Then rtrA goes down. rtrB then becomes active. At that point I do not follow your scenario. I am not sure whether you assume that rtrA comes back into service or if you assume that there is rtrC which became standby.
If rtrA comes into service again and it has higher priority and it has preempt then rtrA becomes active and rtrB returns to standby. This is because preempt on rtrA allows it to displace rtrB. There is some very brief period of transition. I am not sure how long but I know it is quite short.
If rtrB became active and there is rtrC who becomes standby and then rtrB fails then rtrC becomes active and there is no period of interruption.
3)I have the same problem with this description. You describe active and standby, you assume that the active fails, and then you assume that the active fails again without specifying whether the original active router came back in service again.
So let me go back to my example of rtrA and rtrB. Assume that rtrA has higher priority and becomes active and does not have preempt. Assume that rtrB has lower priority and does have preempt. Then rtrA goes down. rtrB becomes active. (Note that this really has nothing to do with preempt - rtrB takes over because the active failed.) If rtrA comes into service then it is in the HSRP group and it has higher priority but it does not become active because it does not have preempt.
Preempt really has to do with enabling a router that has higher priority but is not active to displace the current active router. Preempt is a factor when some router attempts to displace the current active router. It is not a factor when the active router fails.
Let me describe another example. Assume rtrA has priority of 105 and rtrB has priority of 100. So rtrA becomes the active router. Then assume that rtrA is tracking interface serial0/0 and the interface goes down. Now rtrA has priority 95 and rtrB has priority 100. Will rtrB become active? With preempt configured it will and without prempt configured it will not.
HTH
Rick