Things I Should Have Known – HSRP

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I use HSRP all the time.  I work in an environment that’s almost 100% Cisco so it’s a protocol that we seem almost every day.  And while I know it’s Cisco proprietary, it’s still my protocol of choice for first hop redundancy.  So now on to the things I didn’t know….

The Virtual MAC
There have been a few times I’ve been troubleshooting a issue and it would have been handy to know if the MAC I was talking to was an interface or a HSRP virtual.  Somehow, I never knew that all HSRP virtual MACs were of the same format.

0000.0C07.ACXX  (Where XX is the hash for the group number defined)

While not totally something I should have known by now, it is an interesting concept.  MHSRP is the concept of using redundant virtual gateways to spread the load between the routers.  For instance, each router interface would have two standby groups.  Each router would be active for one of them.  In the case of a failure, the other router would assume both.  Now, this relies on the fact that the clients using the HSRP addresses can be split up amongst the two virtual addresses.  I’m thinking of some possible uses when we deploy redundant remote routers though.

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