10.6 OSPF: Open Shortest Path First

OSPF is a newer alternative to RIP as an interior gateway protocol. It overcomes all the limitations of RIP. OSPF Version 2 is described in RFC 1247 [Moy 1991].

OSPF is a link-state protocol, as opposed to RIP, which is a distance-vector protocol. The term distance-vector means the messages sent by RIP contain a vector of distances (hop counts). Each router updates its routing table based on the vector of these distances that it receives from its neighbors.

In a link-state protocol a router does not exchange distances with its neighbors. Instead each router actively tests the status of its link to each of its neighbors, sends this information to its other neighbors, which then propagate it throughout the autonomous system. Each router takes this link-state information and builds a complete routing table.

From a practical perspective, the important difference is that a link-state protocol will always converge faster than a distance-vector protocol. By converge we mean stabilizing after something changes, such as a router going down or a link going down. Section 9.3 of [Perlman 1992] compares other issues between the two types of routing protocols.

OSPF is different from RIP (and many other routing protocols) in that OSPF uses IP directly. That is, it does not use UDP or TCP. OSPF has its own value for the protocol field in the IP header (Figure 3.1).

Besides being a link-state protocol instead of a distance-vector protocol, OSPF has many other features that make it superior to RIP.

OSPF can calculate a separate set of routes for each IP type-of-service (Figure 3.2). This means that for any destination there can be multiple routing table entries, one for each IP type-of-service.
Each interface is assigned a dimensionless cost. This can be assigned based on throughput, round-trip time, reliability, or whatever. A separate cost can be assigned for each IP type-of-service.
When several equal-cost routes to a destination exist, OSPF distributes traffic equally among the routes. This is called load balancing.
OSPF supports subnets: a subnet mask is associated with each advertised route. This allows a single IP address of any class to be broken into multiple subnets of various sizes. (We showed an example of this in Section 3.7 and called it variable-length subnets. ) Routes to a host are advertised with a subnet mask of all one bits. A default route is advertised as an IP address of 0.0.0.0 with a mask of all zero bits.
Point-to-point links between routers do not need an IP address at each end. These are called unnumbered networks. This can save IP addresses ” a scarce resource these days!
A simple authentication scheme can be used. A cleartext password can be specified, similar to the RIP-2 scheme (Section 10.5).
OSPF uses multicasting (Chapter 12), instead of broadcasting, to reduce the load on systems not participating in OSPF.

With most router vendors supporting OSPF, it will start replacing RIP in many networks.