Chapter 9. Open Shortest Path First

Chapter 9. Open Shortest Path First

• Operation of OSPF

  Neighbors and Adjacencies

  Areas

  The Link State Database

  The Route Table

  Authentication

  OSPF Over Demand Circuits

  OSPF Packet Formats

  OSPF LSA Formats

  The NSSA External LSA

  The Options Field

• Configuring OSPF

  Case Study: A Basic OSPF Configuration

  Case Study: Setting Router IDs with Loopback Interfaces

  Case Study: DNS Lookups

  Case Study: OSPF and Secondary Addresses

  Case Study: Stub Areas

  Case Study: Totally Stubby Areas

  Case Study: Not-so-Stubby Areas

  Case Study: Address Summarization

  Case Study: Authentication

  Case Study: Virtual Links

  Case Study: OSPF on NBMA Networks

  Case Study: OSPF Over Demand Circuits

• Troubleshooting OSPF

  Case Study: An Isolated Area

  Case Study: Misconfigured Summarization

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 open ”that 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, was first specified in RFC 1247, and the most recent specification is RFC 2328. ^[1]

^[1] RFC 2328 was released as this chapter was being written, and obsoletes RFC 2178.

Like all link state protocols, OSPF's major advantages over distance vector protocols are fast reconvergence, support for much larger internetworks, 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 internetwork 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 ^[2]

  ^[2] 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. Cisco's OSPF implementation performs equal-cost load balancing as described in previous chapters.

• The use of reserved multicast addresses to reduce the impact on non-OSPF-speaking 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.