Re: HSRP on a single router

bilalzubair · ‎05-12-2005

Can you please let me know if I can configure HSRP on a single router with two ethernet interfaces.

Thanks

dgahm · ‎05-12-2005

No, you cannot use the same IP subnet on more than one interface in a router.

bilalzubair · ‎05-12-2005

Just our of curosity I want to know why, becuase technically if I see its two different interfaces connected to two different networks. Then why CANT I do it on the same router and CAN do it on the different router.

Kevin Dorrell · ‎05-12-2005

Maybe we misunderstood the question. If you have a router with two different Ethernet interfaces, on two different networks, configured to two different IP subnets, then yes, you can run HSRP on each one independently.

If your question means "can I configure HSRP so that two interfaces can operate HSRP for each other on the same network and subnet", then no, you cannot. The reason is that the router will not allow you to configure two interfaces in the same subnet, 'cos otherwise it would not know which one to route to. Yes, I suppose you could argue that it should route to the active one. I cannot think of any reason why they didn't implement that, but they didn't.

I guess you could achieve the same sort of thing by bridging the two interfaces, connecting them to the same network, routing through a BVI, and letting STP sort out the redundancy, but I have never tried it like that.

Hope this helps.

Kevin Dorrell

Luxembourg

b-watkins · ‎05-13-2005

For what it's worth, I have tried that exact thing on more than one occasion and it works perfectly. The failover was relatively quick, as well, though not as quick as if Rapid Spanning-Tree were supported on routers (hint, hint Cisco).

bilalzubair · ‎05-13-2005

Thanks for that I would love to try that, Can you send me hte link of some configuration examples like that..

Regards

Kevin Dorrell · ‎05-13-2005

I don't have any specific links using this technique for redundancy, but in effect it is a standard IRB bridge - you just connect both interfaces to the same network. The Spanning Tree will take care of the redundancy automatically.

Here are a few commands to play with:

interface Ethernet 0

desc Connect this to LAN

no ip address

bridge-group 1

interface Ethernet 1

desc Connect this to LAN as well

no ip address

bridge-group 1

interface bvi 1

desc This interface that will provide your routing

ip address 192.168.42.254 255.255.255.0

bridge irb

bridge 1 protocol ieee

! This will start your Spanning Tree - do it before connecting!

bridge 1 route ip

! Not sure whether you need this command!

Good luck

Kevin Dorrell

Luxembourg

bilalzubair · ‎05-13-2005

Thanks..