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Troubleshooting OSPF

Troubleshooting OSPF Neighbor Relationships

OSPF neighbor relationship problems can be of any of these types:

The OSPF neighbor list is empty.
An OSPF neighbor is stuck in ATTEMPT.
An OSPF neighbor is stuck in INIT.
An OSPF neighbor is stuck in 2-WAY.
An OSPF neighbor is stuck in EXSTART/EXCHANGE.
An OSPF neighbor is stuck in LOADING.
Problem: OSPF Neighbor List Is Empty

The most common possible causes of this problem are as follows:

OSPF is not enabled on the interface.
Layer 1/2 is down.
The interface is defined as passive under OSPF.
An access list is blocking OSPF Hellos on both sides.
A subnet number/mask has been mismatched over a broadcast link.
The Hello/dead interval has been mismatched.
The authentication type (plain text versus MD5) has been mismatched.
An authentication key has been mismatched.
An area ID has been mismatched.
Stub/transit/NSSA area options have been mismatched.
An OSPF adjacency exists with secondary IP addressing.
An OSPF adjacency exists over an asynchronous interface.
No network type or neighbor is defined over NBMA (Frame Relay, X.25, SMDS, and so on).
The frame-relay map/dialer map statement is missing the broadcast keyword on both sides
Problem: OSPF Neighbor Stuck in INIT

The most common possible causes of this problem are as follows:

An access list on one side is blocking OSPF Hellos.
Multicast capabilities are broken on one side (6500 switch problem).
Authentication is enabled on only one side (virtual link example).
The frame-relay map/dialer map statement on one side is missing the broadcast keyword.
Hellos are getting lost on one side at Layer 2.

Problem: OSPF Neighbor Stuck in 2-WAY—Cause: Priority 0 Is Configured on All Routers

Problem: OSPF Neighbor Stuck in EXSTART/EXCHANGE

The most common possible causes of this problem are as follows:

Mismatched interface MTU
Duplicate router IDs on neighbors
Inability to ping across with more than certain MTU size
Broken unicast connectivity because of the following:
- Wrong VC/DLCI mapping in Frame Relay/ATM switch

- Access list blocking the unicast

- NAT translating the unicast

Network type of point-to-point between PRI and BRI/dialer
Problem: OSPF Neighbor Stuck in LOADING

The most common possible causes of this problem are as follows:

Mismatched MTU
Corrupted link-state request packet
Troubleshooting OSPF Route Advertisement

The most common reasons for OSPF to not share the database information about a specific link are as follows:

The OSPF neighbor is not advertising routes.
The OSPF neighbor (ABR) is not advertising the summary route.
The OSPF neighbor is not advertising external routes.
The OSPF neighbor is not advertising the default route.
Problem: OSPF Neighbor Is Not Advertising Routes

The most common possible causes of this problem are as follows:

OSPF is not enabled on the interface that is supposed to be advertised.
The advertising interface is down.
The secondary interface is in a different area than the primary interface.
Problem: OSPF Neighbor (ABR) Not Advertising the Summary Route

The most common possible causes of this problem are as follows:

An area is configured as a totally stubby area.
An ABR is not connected to area 0.
A discontiguous area 0 exists.

Problem: OSPF Neighbor Is Not Advertising External Routes

The most common possible causes of this problem are as follows:

The area is configured as a stub or NSSA.
The NSSA ABR is not translating Type 7 into Type 5 LSA.
Problem: OSPF Neighbor Not Advertising Default Routes

The most common possible causes for an OSPF router not to advertise the default route are as follows:

The default-information originate command is missing.
The default route is missing from the neighbor’s routing table.
A neighbor is trying to originate a default into a stub area.
The NSSA ABR/ASBR is not originating the Type 7 default.

Troubleshooting OSPF Route Installation

The most common reasons for OSPF failing to install routes in the routing table are as follows:

OSPF is not installing any routes in the routing table.
OSPF is not installing external routes in the routing table.

Problem: OSPF Not Installing Any Routes in the Routing Table

The most common possible causes of this problem are as follows:

The network type is mismatched.
IP addresses are flipped in dual serial-connected routers or a subnet/mask mismatch has occurred.
One side is a numbered and the other side is an unnumbered point-to-point link.
A distribute list is blocking the routes’ installation.
There is a broken PVC in a fully meshed Frame Relay network with the broadcast network type.

Problem: OSPF Not Installing External Routes in the Routing Table

The most common causes of this problem are as follows:

The forwarding address is not known through the intra-area or interarea route.
The ABR is not generating Type 4 summary LSAs.

Troubleshooting Redistribution Problems in OSPF

The following are problems that can happen during redistribution:

ASBR is not advertising redistributed routes.
OSPF is not installing external routes in the routing table.

Problem: OSPF Neighbor Is Not Advertising External Routes

The most common causes of this problem are as follows:

The subnets keyword is missing from the ASBR configuration.
distribute-list out is blocking the routes.

Troubleshooting Route Summarization in OSPF


OSPF can use two types of summarization:

Interarea summarization that can be done on the ABR
External summarization that can be done on the ASBR
Two common problems related to summarization in OSPF are as follows:

A router is not summarizing interarea routes.
A router is not summarizing external routes.
Problem: Router Is Not Summarizing Interarea Routes

—Cause: area range Command Is Not Configured on ABR

Problem: Router Is Not Summarizing External Routes

—Cause: summary-address Command Is Not Configured on ASBR

Troubleshooting CPU HOG Problems

The CPUHOG messages usually appear in two significant stages:

Neighbor formation process
LSA refresh process
This section discusses the possible solutions for these two instances of SPF:

CPUHOG messages during adjacency formation
CPUHOG messages during LSA refresh period
Problem: CPUHOG Messages During Adjacency Formation

—Cause: Router Is Not Running Packet-Pacing Code

Problem: CPUHOG Messages During LSA Refresh Period

—Cause: Router Is Not Running LSA Group-Pacing Code

Troubleshooting SPF Calculation and Route Flapping

The problem of SPF running constantly in the network for the following reasons:

Interface flap within the network
Neighbor flap within the network
Duplicate router ID
Common OSPF Error Messages

“Unknown routing protocol” Error Message = the software or the hardware does not support OSPF

OSPF: “Could not allocate router id” Error Message =

This message appears in two situations:

No up/up interface with a valid IP address
Not enough up interfaces with a valid IP address for multiple OSPF processes

“%OSPF-4-BADLSATYPE: Invalid lsa: Bad LSA type” Type 6 Error Message

This is normal if the neighboring router is sending the multicast OSPF (MOSPF) packet. For more information on MOSPF, refer to RFC 1584. Cisco routers do not support MOSPF, so they simply ignore it

ignore lsa mospf

“OSPF-4-ERRRCV” Error Message

Three common types of this message can occur:

Mismatch area ID
Bad checksum
OSPF not enabled on the receiving interface
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