[nsp] HSRP

[Peter B. Juul]

On Thu, Jun 19, 2003 at 08:29:37PM +0200, mac wrote:

> can one give me a quick explanation of HSRP is and how works?

Sure. HSRP, Hot Standby Router Protocol.

The concept is that you have two routers connected to the same Layer 2
network, say 10.1.0.2/24 and 10.1.0.3/24.

You configure the hosts on the net to use 10.1.0.1 as their default
gateway and you then configure HSRP with a virtual IP address of 10.1.0.1.

The routers then decide amongst each other which of them is to be 10.1.0.1
now and the hosts use that router.

Should that router die a horrible death (or should you decide to shutdown
the interface, yank the wire or somesuch) the other router notices and 
quickly takes over the IP address and (as far as I remember) the MAC address
so the ARP tables doesn't make a fuss. 

You can set up priorities for the routers and you can ask them to pre-empt,
so that when the first router is back on line, it takes over again.

It works a charm to my experience.

(It is, afaik, the Cisco version of that which became VRRP. However, VRRP
uses unicast for checking the status whereas HSRP uses multicast. Please,
someone correct me if that's pure nonsense.)

Peter B. Juul, 
Uni·C (PBJ255-RIPE)