Overview of OSPF

Definition

The Open Shortest Path First (OSPF) protocol, developed by the Internet Engineering Task Force (IETF), is a link-state Interior Gateway Protocol (IGP).

At present, OSPF Version 2, defined in RFC 2328, is intended for IPv4; OSPF Version 3, defined in RFC 2740, is intended for IPv6. Unless otherwise stated, OSPF stated in this document refers to OSPF Version 2.

Purpose

Before the emergence of OSPF, the Routing Information Protocol (RIP) is widely used on networks as an IGP.

RIP is a routing protocol based on the distance vector algorithm. Due to its problems of slow convergence, routing loops, and poor scalability, RIP is gradually replaced by OSPF.

As a link-state protocol, OSPF can solve many problems encountered by RIP. Additionally, OSPF features the following advantages:

• Receives and sends packets in multicast mode to reduce load on routers that do not run OSPF.

• Supports Classless Inter-domain Routing (CIDR).

• Supports load balancing among equal-cost routes.

• Supports packet encryption.

With the preceding advantages, OSPF is widely accepted and used as an IGP.