Ospf Overview; Introduction To Ospf; Chapter 4 Ospf Configuration; Process Of Ospf Route Calculation - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide

Chapter 4 OSPF Configuration

4.1 OSPF Overview

4.1.1 Introduction to OSPF

OSPF (Open Shortest Path First) is a link state-based internal gateway protocol
developed by IETF organization. At present, OSPF version 2 (RFC2328) is used, which
allows of the following features:
Applicable scope: It can support networks in various sizes and can support several
hundred 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.

4.1.2 Process of OSPF Route Calculation

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 other. 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 map, which actually reflects the topology architecture of the
whole network. Obviously, all the routers get a map exactly the same.
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Chapter 4 OSPF Configuration