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HSRP active timer question

From Cisco.com

Active Timer: This timer is used to monitor the active router. This timer                     starts any time an active router receives a hello packet. This timer expires in                     accordance with the hold time value that is set in the related field of the                     HSRP hello message.

Can someone verify my assumptions are correct and answer my question

I believe a router within the HSRP group (such as a listening router) would start its Active timer each time it recieved a hello packet from the active router, and start its standby timer each time it recieved a hello from the standby timer.. is this correct?.. and if so my question is if multiple listening routers exist how do they know which one becomes the new standby (should the existing standby fail) is an election forced? and if so is the election just between the existing listening routers?

Thanks in advance

  • LAN Switching and Routing
1 REPLY

Re: HSRP active timer question

Hi,

I believe a router within the HSRP group (such as a listening router) would start its Active timer each time it recieved a hello packet from the active router, and start its standby timer each time it recieved a hello from the standby timer.. is this correct?

From initializing HSRP to the final state (Active, Standby, Listen) there are several events which (re-)start the Active- or Standby-timers and the expiration of those timers also trigger actions. You can see those events and actions in the HSRP state machine:

RFC 2281 5.7 State Transitions

Understanding and Tshoot HSRP...

It takes some time to get used to this kind of presentation, therefore I attached a (very small) partial flowchard to give you a basic idea.

If you have more than two HSRP-enabled routers on a segement, one active and one standby router will be elected and the others will stay in Listen state until the active or standby router fail or parameters are changed (e.g. priorities when preemtion is enabled). There is a special message type called "Resign", which the active router sends when HSRP is disabled on an interface. As a last resort, the other routers will realize the loss of active or standby router based on the timers (absence of hellos).

HTH

Rolf

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