Troubleshooting OSPF Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

Open Short Path First Protocol Overview

1. OSPF data structures store operational data, configured parameters and statistics
2. There are four OSPF data structures. These are:

1. The Interface Table
2. The Neighbor Table
3. The Link State Database
4. The Routing Information Base

1. The Interface Table provides a list of all interfaces that have been enabled for OSPF
2. The Neighbor Table tracks all active OSPF neighbors
3. The Link State Database (LSDB) contains information about the network topology
4. The Routing Information Base (RIB) contains the results derived from the SPF calculation
5. OSPF is a hierarchical routing protocol that logically divides the network into sub-domains
6. This logical segmentation is used to limit the scope of LSA flooding
7. In a multi-area OSPF network, one area must be designated as the backbone area
8. The backbone is the logical center of the OSPF network
9. All other non-backbone areas must be physically or logically connected to the backbone
10. OSPF uses different default network types for different media. These are:

1. Non-Broadcast
2. Point-to-Point
3. Broadcast
4. Point-to Multipoint

1. OSPF neighbors go through several states before the adjacency becomes FULL
2. Valid OSPF states are Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading and Full
3. These parameters must match before an adjacency will go into the Full state:

1. The interface MTU values
2. The Hello and Dead Timers
3. The Area ID
4. The Authentication Type and Password
5. The Stub Area flag
6. IP Subnet and Subnet Mask

1. Each LSA begins with a standard 20-byte LSA header that contains the following:

1. Link State Age
2. Options
3. Link State Type
4. Link State ID
5. Advertising Router
6. Link State Sequence Number
7. Link State Checksum
8. Length

1. Type 1 LSAs are generated by each router for each area it belongs to
2. Type 2 LSAs advertise the routers on the multi-access segment
3. Type 3 LSAs are a summary of destinations outside of the local area
4. Type 4 LSAs describe information about an Autonomous System Boundary Router
5. Type 5 LSAs provide the network information necessary to reach the external networks
6. Type 7 LSAs are used for external routing information from the ASBR in an NSSA
7. OSPF supports addition special types of areas in addition to area 0 and normal areas
8. These special types of areas are:

1. Not-so-stubby Areas
2. Totally Not-so-stubby Areas
3. Stub Areas
4. Totally Stubby Areas

1. A virtual link is a logical extension of the OSPF backbone

Troubleshooting Neighbor Relationships

1. OSPF neighbor relationship issues fall into one of the following categories:

1. The Neighbor Table is Empty
2. The Neighbor is stuck in the ATTEMPT state
3. The Neighbor is stuck in the INIT state
4. The Neighbor is stuck in the 2WAY state
5. The Neighbor is stuck in the EXSTART/EXCHANGE state
6. The Neighbor is stuck in the LOADING state

1. The OSPF Neighbor Table may be empty due to one of the following:

1. Basic OSPF Misconfigurations
2. Layer 1 and Layer 2 Issues
3. Access List Filtering
4. Interface Misconfigurations

• The OSPF adjacency may be stuck in the ATTEMPT state due to the following:

1. Incorrect NBMA Configuration
2. Incorrect OSPF Configuration
3. ACL Filtering
4. Unidirectional Connectivity

1. The OSPF adjacency may be stuck in the INIT state due to the following:

1. ACL Filtering on One Side
2. Layer 1 and Layer 2 Issues
3. NBMA Misconfigurations

1. The OSPF adjacency may be stuck in the 2WAY state due to the following:

1. Neighbors all have their priority values set to zero (0)

1. The OSPF adjacency may be stuck in the EXSTART/EXCHANGE state due to the following:

1. Mismatched MTU Values
2. Duplicate RIDs
3. Broken Unicast Connectivity

1. The OSPF adjacency may be stuck in the LOADING state due to the following:

1. Corrupted LSR packets – which could be due to hardware or software issues