3com 8807 User Guide

Open Shortest Path First (OSPF) is an Interior Gateway Protocol based on the link 
state developed by IETF. At present, OSPF version 2 (RFC2328) is used, which is 
available with the following features:

■

Applicable scope: It can support networks in various sizes and can support 
several hundreds of routers at maximum.

■

Fast convergence: It can transmit the update packets instantly after the 
network topology changes so that the change is synchronized in the AS.

■

Loop-free: Since the OSPF calculates routes with the shortest path tree 
algorithm according to the collected link states, it is guaranteed that no loop 
routes will be generated from the algorithm itself.

■

Area partition: It allows the network of AS to be divided into different areas for 
the convenience of management so that the routing information transmitted 
between the areas is abstracted further, hence to reduce the network 
bandwidth consumption.

■

Equal-cost multi-route: Support multiple equal-cost routes to a destination.

■

Routing hierarchy: OSPF has a four-level routing hierarchy. It prioritizes the 
routes to be intra-area, inter-area, external type-1, and external type-2 routes.

■

Authentication: It supports the interface-based packet authentication so as to 
guarantee the security of the route calculation.

■

Multicast transmission: Support multicast address to receive and send packets.

The routing calculation process of the OSPF protocol is as follows:

■

Each OSPF-capable router maintains a Link State Database (LSDB), which 
describes the topology of the whole AS. According to the network topology 
around itself, each router generates a Link State Advertisement (LSA). The 
routers on the network transmit the LSAs among them by transmitting the 
protocol packets to each others. Thus, each router receives the LSAs of other 
routers and all these LSAs compose its LSDB.

■

LSA describes the network topology around a router, so the LSDB describes the 
network topology of the whole network. Routers can easily transform the LSDB 
to a weighted directed graph, which actually reflects the topology architecture 
of the whole network. Obviously, all the routers get a graph exactly the same.

■

A router uses the SPF algorithm to calculate the shortest path tree with itself as 
the root, which shows the routes to the nodes in the autonomous system. The 
external routing information is the leave node. A router, which advertises the 
routes, also tags them and records the additional information of the