What our viewers down under may not know, is that Paul Hogan has been very busy here in the U.S. promoting the Australian tour and vacation industry. I mean, it’s relentless. Everywhere you turn, there’s Paul and his smelly little friend, Vic the Koala. Americans love Australia and having Paul Hogan up here, dragging this flea bitten, disagreeable koala from place to place is so unnecessary. Every day the television news shows another innocent American getting fiercely attacked by this rabid koala as Hogan struggles to gain control of the ravenous beastie. And you Aussies are living the good life down in OZ unaware of the carnage that Hogan and his maniacal, blood thirsty koala are wreaking up here in the ‘States. We love Australia, we forgave you for Helen Reddy years ago. We promise to come visit, but please, please make Hogan stop! We met Hogan here in Los Angeles at an autograph signing where the LAPD had to restore order after “Vic the Koala” went on another one of his trademark rampages. Hogan was so shaken and apologetic that he agreed to do an interview about Cisco’s Hot Standby Router Protocol. Let’s join RouterGod reporter Doug Leong as he talks to the star of those cool Crocodile Dundee movies, Paul Hogan:



Paul tries to stay upbeat amidst
controversy surrounding “Vic the Koala”.


Paul Hogan poses with 
psychopathic koala.
Doug

Paul, welcome to RouterGod Online Magazine, it’s great to talk with you.

Paul

No worries mate, say, I’m sorry about the business between Vic and the police, sometimes the little devil gets a might cranky.

Doug

Well everything seems to be OK now, why don’t you explain HSRP to our audience?

Paul

OK (clears throat) let’s talk about the Hot Standby Router Protocol.  HSPR is a way for two Cisco routers to share a common virtual IP address while one router is actively routing packets, the other router or routers are standing by in case the active router fails.  It’s a very good way to provide fault tolerance.  In fact, if ya take a mind to, you can read RFC 2281 to learn a bit more about it.

Doug

So the routers share an IP address.  How do the routers know when one has failed?

Paul

Let’s get back to the IP address, besides sharing an IP address, that IP address has a common MAC address that the routers share.  It’s like this, mate, you have a workgroup of say, I don’t know, 100 computers.  Each one of these machines has been configured with a default gateway, if these machines have used the default gateway or router, they have it’s MAC address in their ARP cache.  So since the routers in the HSRP group share an IP address with a corresponding MAC address, when they fail over, the workstations are unaware of the change.  What they see, is a “virtual” router.

Doug

How do the routers control all this?

Paul

Well mate, the routers in an HSRP group send and receive keep alives using the multicast address of 224.0.0.2 and UDP port 1985.  By default the hello interval is 3 seconds.  Once 3 hello intervals pass without hearing from the active router, the standby router automatically becomes the active router.  Each router is configured with a priority number, the router with the highest priority number in a standby group is the active router, everyone else just relaxes.

Doug

This must be very hard to configure…

Paul

No way mate, you only need 2 commands to do it, and 2 additional commands to customize it.  What’s more, it’s configured at the interface that you want to participate in the standby group.  It’s so easy, an American could do it!

Doug

No way!  Really?  What are the commands?

Paul

Well, first off, on the router that you want to be the active router, go to the interface you want HSRP to run on, think up a group number, all routers participating in this scheme must use the same group number, also think up what IP address you want the HSRP group to share.  Now watch me type:

dingo(config-if)#standby 1 ip address 10.1.1.254
dingo(config-if)#standby 1 priority 100

So what we have here mate, is a standby group number of 1, an IP address of 10.1.1.254 that the routers are going to share, now let’s configure the standby router:

fosters(config-if)#standby 1 ip address 10.1.1.254
fosters(config-if)#standby 1 priority 90

OK, the only thing different on the standby router is the priority.  The router with the highest priority becomes the active router.

Doug

What if the active router, with the priority of 100 goes down and comes back up, since it has the higher priority, will it become active again?

Paul

Sorry mate, in that situation, if you want it to be the active router again, you have to add the keyword preempt to the priority command:

dingo(config-if)#standby 1 priority 100 preempt

Doug

Hey Paul, what if the routers are connected to a WAN link, and the routers are running fine, but the WAN link goes down, is there any way to track that?

Paul

You said it my clever friend, the command is track.  Say you want to have HSRP failover if the WAN link goes down, well just track the interface that the WAN link is connected to, like so:

dingo(config-if)#standby 1 track s0 priority 11

Noticed that there is the keyword priority followed by a number.  That number is the number to subtract from the router’s own priority number to give it an adjusted priority number if the interface it’s tracking goes down.  If the Serial 0 interface goes down, our router’s priority goes from 100 to 89 which will cause the standby router to become active since it has a priority of 90.  You might need this to happen if the standby router is configured for DDR.

Doug

This has all been very interesting, welcome to America and thanks for explaining HSRP!

Paul

No worries mate!


Visit the HSRP Guide and the HSRP FAQ at Cisco.  Also see Cisco’s exciting page on Configuring HSRP. Remember, never charge less than $1000 to set up HSRP and always get the money up front, before your customer sees that you fully configured HSRP on 2 routers in less than 5 min!


 

 

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