Chapter 9: Troubleshooting OSPF Routing Environments

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Open Shortest Path First (OSPF) is an industry standard protocol that ensures interoperability between routing devices manufactured by diverse vendors. OSPF is a complex link state protocol with concepts of adjacency, areas, incremental updates, and authentication.

This chapter discusses different methods of problem isolation and troubleshooting in OSPF networks.

OSPF Terminology

This section discusses the terminology used when discussing the OSPF routing protocol. Some of the terms are:

Link State Advertisement (LSA): OSPF packet containing source, destination, and routing information. This information is advertised to all OSPF routers in a hierarchical area.

Flooding: Periodic updating of topology and routing table information with sending of LSAs.

Adjacencies: Logical connection between OSPF router and its Designated Router (DR).

Designated Router: Router used to reduce the number of adjacencies formed in a broadcast network.

Backup Designated Router (BDR): Router that acts as a standby for DR on broadcast networks. BDR collects routing information updated from the adjacent OSPF routers and takes the role of DR when the DR goes down.

Autonomous Systems (ASs): Set of routers in the same administrative control using the same protocol for routing processes.

Multi-access/Broadcast Networks: Physical networks that support interconnection of more than two routers that can communicate directly.

Nonbroadcast Multi-access Networks (NBMAs): Interconnect routers in an OSPF network without having the broadcast capability. NBMA is discussed in detail in a later section.

Single Area (SA): Logical subdivision of the greater OSPF domain, grouping routers that run OSPF with identical topological databases.

Stub Area: Type of nonstandard OSPF area.

Totally Stubby Area (TSA): Type of nonstandard OSPF area used when few networks with limited connectivity are connected to the remaining network.

Not So Stubby Area (NSSA): Type of nonstandard OSPF area used in redistribution of routing information.

OSPF Routers: Four types of OSPF routers are Internal Router (IR), Area Border Router (ABR), Backbone Router (BR), and Autonomous System Boundary Router (ASBR).