Cisco ISR and HSRP Configuration - Priorities

[ciscocharger]

Hi,

I was hired to clean up a network and document the topology and confgiuration.

In the environment, there are a pair of Cisco 1900 series ISRs that are configured for HSRP and I'm having some difficulty figuring out why the HSRP is configured the way it is (the person that did the configuration has moved on and no topology documentation exists).

Here are the HSRP relevant sections:

R1:

...

track 101 ip route 10.1.20.0 255.255.255.0 reachability
track 102 ip route 10.1.21.0 255.255.255.0 reachability
track 103 ip route 10.1.22.0 255.255.255.0 reachability

...

interface GigabitEthernet0/0
 ip address 192.168.200.4 255.255.255.0
 standby 1 ip 192.168.200.5
 standby 1 priority 255
 standby 1 preempt delay minimum 10
 standby 1 track 101 decrement 10
 standby 1 track 102 decrement 10
 standby 1 track 103 decrement 10

--------------

R2:

...
track 101 ip route 10.1.20.0 255.255.255.0 reachability
track 102 ip route 10.1.21.0 255.255.255.0 reachability
track 103 ip route 10.1.22.0 255.255.255.0 reachability

...

interface GigabitEthernet0/0
 ip address 192.168.200.6 255.255.255.0
 standby 1 ip 192.168.200.5
 standby 1 priority 115
 standby 1 preempt
 standby 1 track 101 decrement 10
 standby 1 track 102 decrement 10
 standby 1 track 103 decrement 10

--------------

"show standby" indicates that R1 is currently the active.

My assumption is that HSRP is configured with virtual IP "192.168.200.5" and is tracking connectivity to the networks defined by 101, 102 and 103. If any of these networks become unreachable, the standby will pre-empt and become the active.

However, I don't understand why the priority and decrement values are set the way they are. I don't a lot of experience with HSRP and am stumped by this. I've watched a fair amount of videos explaining HSRP config but am not able to reconcile what I understand with the config I'm looking at.

Can anyone explain this config and what it's doing?

Any help is greatly appreciated.

Thanks

M

[balaji.bandi · Accepted solution]

Yes, you are right the decrement value not make any sense for the standby to become active.

the decrement lower value than standby so standby becomes active.

here is the good document so can understand better : (i can offer solution, but I always suggest to understand the person who doing it, so he learn lesson and maintain their environment stable).

https://www.cisco.com/c/en/us/td/docs/switches/blades/3040/software/release/12-2_44_se/configuration/guide/swhsrp.html

[Richard Burts · Accepted solution]

M

Your understanding of what was intended for this configuration is correct. And you are correct that the settings of priority and of decrement defeat that intention. With a priority of 255 and decrement of 10 R1 will never have a priority that would allow R2 to preempt and to take over as active router. If R1 fails then R2 can become the active router. 

[ciscocharger]

Thanks everyone for your answers.

Greatly appreciated!

M