Subject: Question on HSRP
> Does HSRP work at the interface level or is the entire router on
> acvtive/stand-by?
HSRP operates at the interface level.
The benefit HSRP provides is this: Let's say you have a number of Workstations
or Servers on a LAN segment where more than one router exist. Each
station/server needs a default gateway in order to communicate with non-local
points in your network. Generally speaking, you can only specify a single
default gateway...
If you assign the RouterA ethernet address (192.168.1.1) as the default gateway
for the servers, and something happens to RouterA, your servers will not know to
send outbound traffic to RouterB's ethernet address (192.168.1.2) instead.
The solution to this OS limitation is to set up HSRP for RouterA and RouterB,
having them each monitor 192.168.1.3. One of the routers will be actively
answering for the .3 HSRP address (You can adjust the settings if you have a
perference), and the other will be standing by, periodically checking to see
that RouterA is still able to accept traffic destined for .3.
If RouterB determines that RouterA has gone missing, RouterB will go active, and
start answering for any traffic destined for .3. (RouterB determines that
RouterA has failed via a "Hello-type" mechanism.)
If you configure RouterA with the 'preempt' option, it will resume answering the
.3 address when it is able to, and RouterB will return to standby mode.
This is all HSRP does... It has no part in path determination, and it
completely seperate from your routing protocol. It simply ensures that devices
on the LAN segment have someplace to send non-local traffic.
Often, the Active HSRP router will simply re-direct the traffic it accepts to
another router which may be running as a standby, depending on your topology,
and your routing tables.
On some operating systems, you can run a routing protocol if you want; Unix
systems in particular. If you were to do that, there would be no need for HSRP,
as each host would have complete routing information. They'd recover as the
absence of a router was discovered, and the network reconverged.
Hope this helps,
Alan
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