How can I configure multiple default gateways on a CISCO Router?

Solution 1:

IIRC, if you put different metrics only the best route will be injected in the main routing table.

So you have three choices:

First choice, as @dkaragasidis said, don't put the metrics (to get per session load-balancig) but then the traffic will flow nearly 50% on both links

Second choice, use some sort of dynamic routing protocol (i suggest BGP) if possbile.

Third choice, use ip sla commands to costantly check for ip availability.

Keep in mind that if one FastEthernet interface (or the remote port with which is connected) fails (i.e. the status of the interface goes to down) then the router automatically injects the other default route, but NOT if the router somehow doesn't work (in that case the ip sla commands will take care of this).

If BGP is not an option and you really don't wanto to balance the gateways, ip sla is your best solution. Take a look at this answer to a similar question for a link to some examples.

Solution 2:

Consider checking out Cisco's Configuring a Gateway of Last Resort Using IP Commands.

If you use multiple ip route commands to configure a default route, traffic is load-balanced over the multiple routes.