Metrics

A router's route table contains the best route to each destination network it knows about.

If a router using a dynamic routing protocol learns two different routes to the same destination, how does it determine which is best?

It uses the metric value of the routes to determine which is best. A lower metric = better.

Each routing protocol uses a different metric to determine which route is the best.

If a router learns two (or more) routes via the same routing protocol to the same destination (same network address, same subnet mask) with the same metric, both will be added to the routing table. Traffic will be load-balanced over both routes.

ECMP (Equal Cost Multi-Path)

ECMP with static routes

IGP	Metric	Explanation
RIP	Hop count	Each router in the path counts as on 'hop'. The total metric is the total number of hops to the destination. Links of all speeds are equal.
EIGRP	Metric based on bandwidth and delay (by default)	Complex formula that can take into account many values. By default, the bandwidth of the slowest link in the route and the total delay of all links in the route are used.
OSPF	Cost	The cost of each link is calculated based on bandwidth. The total metric is the total cost of each link in the route.
IS-IS	Cost	The total metric is the total cost of each link in the route. The cost of each link is not automatically calculated by default. All links have a cost of 10 by default.

OSPF take into acount that R3<->R4 is a FastEthernet connection, while RIP sees all connections as equals.