per-packet load balancing and HSRP

[smith.tom]

I have a Cisco 3725 Router with four serial interfaces and I configured per-packet load balancing on each serial interface. I also have HSRP configured on f0/0.

Is it possible to have HSRP track all four serial interfaces without causing an HSRP failover when one serial interface goes down?

[Richard Burts]

You mentioned per-packet load balancing but I do not see how that relates to the question of HSRP tracking. Did I miss something?

Yes it is possible to configure HSRP to track multiple interfaces and not cause a failover when a single interface goes down. I have done that at a customer site and it worked.

To do this it is important to understand the relationship of HSRP track which lowers the priority of the HSRP interface and how priority is set on an interface. What HSRP track does is to lower the priority of the HSRP interface when an interface goes down. By default it will lower the priority 10 and you can configure a different amount if you want. For this discussion lets assume that it will lower by 10. If you want the failover to happen when one interface goes down make sure that the difference in priority of the interfaces is less than 10. to have the failover happen when two interfaces go down, make sure that the difference in priority of the interfaces is between 11 and 19. When the first interface fails the priority lowers by 10 but the primary interface continues. When a second interface fails it lowers by another 10 (total of 20) and it allows the backup to take over. If you wanted failover to happen when three interfaces fail, make sure that the difference in priority of the interfaces is between 21 and 29.

HTH

Rick

[a.awan]

When you say you have enabled per packet load balancing i hope you are doing it through CEF.

HSRP can be configured to track multiple objects (1 to 500) as of IOS 12.2(15)T. Even prior to that HSRP was able to track multiple interfaces. The concept of objects has added more flexibility to HSRP tracking as now you can actually monitor not only objects but also ip routes and some other parameters. To answer your question, yes it is possible to configure HSRP so that the switchover from active to standby does not happen if only one interface/object goes down.

For example lets assume Router A has HSRP priority 100 and Router B has HSRP priority 90. We configure Router A to track three interfaces and with each interface going down we decrease the priority by 4. This will ensure that if two interfaces go down Router A is still the active as its priority will be 92 which is higher than 90.

[smith.tom]

I understand.

Thanks for straigt to the point answer.