Chapter 8. OSPFv2

This chapter covers the following subjects:

• Operation of OSPF

• Configuring OSPF

• Troubleshooting OSPF

Open Shortest Path First (OSPF) was developed by the Internet Engineering Task Force (IETF) as a replacement for the problematic RIP and is now the IETF-recommended Interior Gateway Protocol (IGP). OSPF is a link-state protocol that, as the name implies, uses Dijkstra's Shortest Path First (SPF) algorithm and that is openthat is, it isn't proprietary to any vendor or organization. OSPF has evolved through several RFCs, all of which were written by John Moy. Version 1 of the protocol was specified in RFC 1131; this version never progressed beyond the experimental stage. Version 2, which is still the current version for IPv4, was first specified in RFC 1247, and the most recent specification is RFC 2328.

Like all link-state protocols, OSPF's major advantages over distance vector protocols are fast reconvergence, scalability to much larger networks, and less susceptibility to bad routing information. Other features of OSPF are

• The use of areas, which reduces the protocol's impact on CPU and memory, contains the flow of routing protocol traffic, and makes possible the construction of hierarchical network topologies

• Fully classless behavior, eliminating such classful problems as discontiguous subnets

• Support of classless route table lookups, VLSM, and supernetting for efficient address management

• A dimensionless, arbitrary metric

• Equal-cost load balancing for more efficient use of multiple paths^[1]

  ^[1] More accurately, the RFC calls for equal-cost multipath, the discovery and use of multiple equal-cost paths, without prescribing how the protocol should route individual packets across these multiple paths. The Cisco OSPF implementation performs equal-cost load balancing as described in previous chapters.

• The use of reserved multicast addresses to reduce the impact on non-OSPFspeaking devices

• Support of authentication for more secure routing

• The use of route tagging for the tracking of external routes

OSPF also has the capability of supporting Type of Service (TOS) routing, although it was never widely implemented. RFC 2328 has deleted the TOS routing option for this reason.