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.
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
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.
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