Preempting on Standby interfaces

[cz96p6]

We have multiple VLANs in our network (network is not functional, creating congfiguration templates)and we are running HSRP on both MFSC in distribution layer. I have configured the preempt command on both active and standby interfaces to force the MSFC's to become active in case the other MSFC goes down. Some of my colleges do not agree with me, I know this a good practice but do not know how to justify my position.

[daniel.bowen]

Preempt allows for the standby interface to become the active interface if the active router is to go down. But it will still need its priority to be higher than all other interfaces. If you do not use the standby preempt command in the router config, then it cannot become the active router should the active router fail.

Daniel,

[MickPhelps]

The preempt command allows an interface to become active when its priority *becomes* higher than the current active interface in an HRSP group.

If two routers are using HSRP and the active interface fails, hello packets will not be received by the standby router and it will become active without preempt.

When the original active router comes back up and the original HSRP interface sees that its priority is higher than the current (original standby) active interface, the only way that it can become active again is if a)preempt is set or b)the current active router fails.

Mick.

[cz96p6]

Mick,

Are you saying that I sould not have preempt command on my standby routers interfaces? so when the active router comes back, he can take his role back from standby router.

e.g

MSFC1

interface Vlan10

description

ip address 10.19.80.195 255.255.255.0

standby 6 priority 200 preempt

standby 6 ip 143.219.80.1

interface Vlan11

description

ip address 10.19.81.195 255.255.255.0

standby 6 priority 200 preempt

standby 6 ip 143.219.81.1

MSFC2

interface Vlan10

description

ip address 10.19.80.196 255.255.255.0

standby 6 priority 100 preempt

standby 6 ip 143.219.80.1

interface Vlan11

description

ip address 10.19.81.196 255.255.255.0

standby 6 priority 100 preempt

standby 6 ip 143.219.81.1

[MickPhelps]

You are correct.

In your case, standby on the active routers could be appropriate so that they can become active again after a failure. Removing preempt from the primary or at least setting a delay will keep HSRP from flapping.

Using preempt on a secondary router is very useful in conjunction with the "standby track" option on the primary. "track" will lower the priority of the HSRP interface; if it falls below the priority of the secondary and the secondary has preempt set, then the secondary will become active.

Mick.