HSRP selecting primary device

beginner_ccna

cant find the answer to this:

how does hsrp select the primary device which is in an active state?

say for example, device one on fa0/0 is configured to 192.168.1.1, priority 100

and device two, fa0/0 is configured to 192.168.1.2, priority 100

is it the lowest ip address?

mattau

by default it will be the device that boots first due to the preempt rule.

if preempt is enabled and priorities are tied it will be the highest ip on the interface.

wave

Hey mattau, I'm not sure that is correct. HSRP seems to preempt only based on priority and not highest IP. See: HSRP vs VRRP preemption - IEOC - INE's Online Community

I haven't labbed this up yet but it appears others have.

wave

"A standby router with equal priority but a higher IP address will not preempt the active router."

First Hop Redundancy Protocols Configuration Guide, Cisco IOS Release 12.4T - Configuring HSRP* [Cisco IOS Software Releases 12.4 T] - Cisco Systems

I just labbed this and if there are three routers all with default settings (default priority of 100 and no preempt). If there is an election because the active goes offline then the Standby router with the highest IP wins.

However, if priorities are tied and preempt is enabled, the highest IP address won't have any effect on which router is Active. Your 'boots first' rule still applies.

mattau

oh really? wow didnt realise that little quirk. ill have to lab that up aswell.

thanks : )

wave

mattau wrote: »

oh really? wow didnt realise that little quirk. ill have to lab that up aswell.

thanks : )

Yeah, I'm doing revision for SWITCH at the moment and when I read your post I remembered readying about this, so it was a good opportunity to dive into the detail again.

vishaw1986

Hey ,

As Default priority is 100. The router with the highest priority value (255 is highest) becomes the active router for the group. If all router priorities are equal or set to the default value, the router with the highest IP address on the HSRP interface becomes the active router. Active router,exchange HSRP hello messages(to the multicast destination 224.0.0.2 using UDP port 1985) at regular intervals and only the standby router monitors the hello messages from the HSRP active router. By default, hellos are sent every 3 seconds.If hellos are missed for the duration of the holdtime timer (default 10 seconds, or three times the hello timer), the active router is presumed to be down. The standby router is then clear to assume the active role. At that point, if other routers are sitting in the Listen state, the next-highest priority router is allowed to become the new standby router.

thedrama

vishaw1986 wrote: »

Hey ,

As Default priority is 100. The router with the highest priority value (255 is highest) becomes the active router for the group. If all router priorities are equal or set to the default value, the router with the highest IP address on the HSRP interface becomes the active router. Active router,exchange HSRP hello messages(to the multicast destination 224.0.0.2 using UDP port 1985) at regular intervals and only the standby router monitors the hello messages from the HSRP active router. By default, hellos are sent every 3 seconds.If hellos are missed for the duration of the holdtime timer (default 10 seconds, or three times the hello timer), the active router is presumed to be down. The standby router is then clear to assume the active role. At that point, if other routers are sitting in the Listen state, the next-highest priority router is allowed to become the new standby router.

wow, you just nicely summarized the topic. However, how about the preempt rule?

pert

thedrama wrote: »

wow, you just nicely summarized the topic. However, how about the preempt rule?

Everything vishaw1986 said is correct, but the most important rule to remember is that without preemption being enabled, the first switch to come up is the active switch, unless it gets preempted or goes down.