When local hosts don't understand multiple local gateways, the set of gateways needs to appear as one device. The Internet Engineering Task Force's Virtual Router Redundancy Protocol (VRRP, RFC 2338) lets you configure a virtual router address that will be taken over by a backup router if the first one fails. VRRP is a local router failover protocol, not an Interior Gateway Protocol (IGP) that learns paths beyond the local subnet.

General route discovery with IGPs

Routers are not restricted to a spanning tree topology, but can use any path they can reach.

Each router needs to learn the next sequence of hops that get you one step closer to the destination, much like knowing a freeway exit says "Baltimore/New York," and subsequent signs will point separately to Baltimore and to New York. Routers can almost instantly fail over to an alternate path, faster but with more complexity than bridges.

In small networks, or for routers at edge customer sites feeding into the network, you really don't need full-fledged dynamic routing. The local router needs to know the hierarchically higher router(s) in the distribution tier. Local routers may just need to know alternate paths to redundant distribution routers. For low-speed installations, this alternate path can be dial-up.