Redundant links between subnets

We have two 1841s. So the connection is between two cisco routers over
two non-cisco routers. I havent tried it but I was under the impression
static routes could be entered for the other cisco router on each
router for the sake of hello packets, and the neighbor command can be
used to specify the neighbor for hello packets. If this doesnt work,
you're right I'll have to resort to GRE tunnels.

I wonder if I can do GRE tunnels just for the hello packets. Another
thing I just thought of was packets in each side will go over to the
other side's cisco router before entering the other side's subnet,
since the destination subnet should be behind the cisco router running
the IGP. I wonder if the hello packet triggers can be used to bring
up/down different routes.

 

Its not quite that simple, because each routing protocol can have
additional conditions that must be met.

EIGRP certainly does and that condition being that neighbours MUST be
addressed from the same subnet which you will not be true in your
particular circumstance.

OSPF will require testing. OSPF may form an adjacency and allow LSA
exchange. The real test is whether it will insert the learned LSA's
into the main routing table given that the neighbours are on two
different subnets. If it does you are homefree. You can then tune the
hello timers as required.

As your stated objective is to make your connectivity more robust, keep
the setup as simple.

And as Vince indicate what out for the MTU hit with GRE tunnels.

Also for GRE tunnels remebber to configure a static route for the
tunnel destination address to prevent recursive lookups.

 

You can do GRE tunnels just for the hello packets BUT...
1- It is ugly and error prone.
2- Even if you get it right, last time I checked Cisco only updates static
routes once per minute unless a link has failed hard, so your average
recovery time will be over 30 seconds and be nowhere close to meeting your
availability requirements.

Been there, done that, been burnt...