This document provides troubleshooting information for common OSPF neighbor and routing issues including: - The OSPF neighbor list being empty and potential causes like interfaces being down, authentication mismatches, etc. - Neighbors being stuck in various states like INIT, 2-WAY, EXSTART/EXCHANGE, and LOADING and potential causes for each. - Routes not being advertised or installed properly and troubleshooting redistribution, summarization, CPU hog, and SPF calculation issues. - Explanations of common OSPF error messages and their meanings.

1. TROUBLESHOOTING OSPF

2. THE 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

3. 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

4. OSPF NEIGHBOR STUCK IN
EXSTART/EXCHANGE
 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

5. OSPF NEIGHBOR STUCK IN LOADING
 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.

6. : OSPF NEIGHBOR (ABR) NOT ADVERTISING
THE SUMMARY ROUTE
 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.

7.  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.

8.  : 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.

9. OSPF NOT INSTALLING EXTERNAL ROUTES
IN THE ROUTING TABLE
 The forwarding address is not known through the intra-
area or inter-area 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,.

10.  OSPF can use two types of summarization: Inter-
area 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 inter-area routes. A router is not
summarizing external routes

11. : ROUTER IS NOT SUMMARIZING INTER-AREA
ROUTES
 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 —

12.  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

13. 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