Advanced Cisco Routing: OSPF Metrics

Suppose there are multiple routes between hosts on a network? How does OSPF choose a preferred route? I'm a big fan of practical experience, so to answer this question, I fired up GNS3 and started building networks to find out:

There are two endpoints, R5 and R6 in this network. There is a core network consisting of R1, R2, R3 and R4 which provide the transport between these two endpoints. Looking at this network diagram, you can see that there are two routes that traffic between these endpoints might take: first, there is the route R1-R2-R3-R4; second, there is the much shorter route of R1-R4. You can make a logical argument for either of these routes being better. From R1, there are two paths to R4, one of which is across a FastEthernet interface, and the other of which is across a 10M Ethernet interface. From R1's perspective, route R1-R2-R3-R4, going through the FastEthernet interface, might appear to be better, since the FastEthernet interface has ten times the bandwidth of the Ethernet interface connecting R1 and R4. However, there are more hops on that route, and the Serial link between R2 and R3 is much slower than the Ethernet interface between R1 and R4. Consequently, a human with the complete picture of the network can see that R5-R1-R4-R6 is clearly the better path. Does OSPF see that, also?

R5#traceroute 192.168.2.2

Type escape sequence to abort.
Tracing the route to 192.168.2.2

1 192.168.1.1 4 msec 4 msec 4 msec
2 172.16.1.2 12 msec 8 msec 8 msec
3 192.168.2.2 28 msec 24 msec *
R5#

Yep. Like a human with a network diagram, OSPF forms a complete picture of the network topology and makes intelligent decisions about the best route between endpoints.

However, suppose the topology were different. What if R1-R4 were connected by a serial link and R1-R2-R3-R4 were all FastEthernet links, like so?:

In this case, there is only one hop on the R1-R4 path, whereas there are four hops on the R1-R2-R3-R4 path. However, the bandwidth across the FastEthernet links is much better than the bandwidth across the serial link. A human would most likely choose R1-R2-R3-R4 to pass the traffic. What does OSPF choose as the best path?
R5#traceroute 192.168.2.2

Type escape sequence to abort.
Tracing the route to 192.168.2.2

1 192.168.1.1 4 msec 8 msec 0 msec
2 172.16.2.2 16 msec 12 msec 12 msec
3 172.16.3.2 24 msec 24 msec 24 msec
4 172.16.4.2 36 msec 24 msec 28 msec
5 192.168.2.2 48 msec 40 msec *
R5#

Once again, OSPF uses its comprehensive view of the network to choose the best path between endpoints. However, if you need to force OSPF to choose a particular path, it is possible to do so by manually assigning a cost to a link, but that's a topic for another post ;)