Friday, 5 February 2010

Cisco Gotchas - 16 Unique Standby Groups

HSRP is configured in standby groups, each one having an identifier value in the switch configuration.

The acceptable range of values is 0-255 and you can create as many as you want. Cisco don't recommended using more than 500 on a 6500 series or 64 on the 3550.

There are limitations on the number of unique ID values you can assign however.


Each instance is configured using the standby command:

interface FastEthernet 0/10
ip address 10.0.0.3 255.255.255.0
standby 10 ip 10.0.0.1
standby 10 preempt


The gotcha is that you can create as many groups as you want, but you can only use 16 different values for the ID.

This is hardware/software specific, but it's not clear what hardware it's specific to. The Cisco FAQ says it only affects PFC2, but their bug tracker doesn't mention the PFC and has a "fixed in" entry indicating that it's not an issue with newer software.

So feel free to create as many unique IDs as you like, but when adding the 17th check the logs to make sure it's actually working!

Or you could use VSS and get rid of HSRP entirely.

The Cisco FAQ for this issue on the 6500 is here.
There is a bug open on the Cisco tracker, CSCdx32496.

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