Overview
Open Shortest Path First (OSPF) is a link state interior gateway protocol developed by the OSPF working group of the IETF. Now, OSPF version 2 (RFC 2328) is used. Unless otherwise noted, OSPF refers to OSPFv2 throughout this chapter. The term "router" in this chapter refers to both routers and Layer 3 switches.
OSPF has the following features:
Wide scope—Supports various network sizes and up to several hundred routers in an OSPF routing domain.
Fast convergence—Transmits routing updates instantly upon network topology changes.
Loop-free—Computes routes with the shortest path first (SPF) algorithm to avoid routing loops.
Area-based network partition—Splits an AS into different areas to facilitate management. In addition, routing information transmitted between areas is summarized to reduce traffic and routing table sizes.
Equal-cost multi-path (ECMP) routing—Supports multiple equal-cost routes to a destination.
Routing hierarchy—Supports a four-level routing hierarchy that prioritizes routes into intra-area, inter-area, external Type-1, and external Type-2 routes.
Authentication—Supports area- and interface-based packet authentication to ensure the security of packet exchange.
Support for multicast—Multicasts protocol packets on some types of links.