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Because it is a link-state routing protocol, OSPF scales well with a growing network. But this
scalability introduces complexity in design, configuration, and maintenance. This section
introduces some of the common issues surrounding an OSPF network and offers a flowchart
approach to troubleshooting these issues.
Components of Troubleshooting OSPF
Troubleshooting OSPF requires an understanding of the operation of the protocol as well as a
specific approach methodology. Figure 4-8 shows the major components of OSPF troubleshooting
and the order in which the process flows.
Figure 4-8 Components of Troubleshooting OSPF
The major components of OSPF troubleshooting include the following:
■ OSPF neighbor adjacencies
■ The OSPF routing table
■ OSPF authentication
Troubleshoot
OSPF Neighbor
Adjacencies
Troubleshoot
OSPF Routing
Table Issues
Troubleshoot
OSPF
Authentication
Troubleshooting OSPF 161
Troubleshooting OSPF Neighbor Adjacencies
The first component to troubleshoot and verify is the OSPF neighbor adjacency. Figure 4-9 shows
the verification/troubleshooting components for neighbor adjacencies.
Figure 4-9 Troubleshooting OSPF Neighbor Adjacencies
A healthy OSPF neighbor state is “Full.” If the OSPF neighbor state remains in any other state, it
may indicate a problem. Example 4-12 demonstrates sample output from the show ip ospf
neighbor command to gather this information.
To determine whether a possible Layer 1 or Layer 2 problem exists with a connection, display the
status of an interface using the show ip ospf neighbor command. “Administratively Down”
indicates that the interface is not enabled. If the status of the interface is not up/up, there will be
no OSPF neighbor adjacencies. In Example 4-13, serial 0/0/1 is up/up.
Example 4-12 Verifying OSPF Neighbor State
RouterX#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.16.31.100 0 Full/ - 00:00:31 10.140.1.1 Serial0/0/0
192.168.1.81 0 Full/ - 00:00:31 10.23.23.2 Serial0/0/1
Troubleshoot
OSPF Neighbor
Adjacencies
Troubleshoot
OSPF Routing
Table Issues
Troubleshoot
OSPF
Authentication
Are all
interface statuses
up/up?
Do local and
neighbor MTU
sizes match?
Are all
required OSPF
neighbors
configured?
Does
all required
hello packet
information match
neighbor?
162 Chapter 4: Single-Area OSPF Implementation
For OSPF to create an adjacency with a directly connected neighbor router, both routers must
agree on the maximum transmission unit (MTU) size. To check the MTU size of an interface, use
the show interface command. In Example 4-14, the MTU size is 1500 bytes.
The network command that you configure under the OSPF routing process indicates which router
interfaces participate in OSPF and determines in which area the interface belongs. If an interface
appears under the show ip ospf interface command, that interface is running OSPF. In Example
4-15, interfaces serial 0/0/1 and serial 0/0/0 are running OSPF.
Example 4-13 Verifying Interface Status
RouterX#show ip ospf interface
Serial0/0/1 is up, line protocol is up
Internet Address 10.23.23.1/24, Area 0
Process ID 100, Router ID 192.168.1.65, Network Type POINT_TO_POINT, Cost: 1562
Example 4-14 Verifying Interface MTU Size
RouterX#show ip interface fa0/0
FastEthernet0/0 is up, line protocol is up
Internet address is 10.2.2.3/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is not set
Example 4-15 Verifying Whether an Interface Is Running OSPF
RouterX#show ip ospf interface
Serial0/0/1 is up, line protocol is up
Internet Address 10.23.23.1/24, Area 0
Process ID 100, Router ID 192.168.1.65, Network Type POINT_TO_POINT, Cost: 1562
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:04
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.168.1.81
Suppress hello for 0 neighbor(s)
Simple password authentication enabled
Serial0/0/0 is up, line protocol is up
Internet Address 10.140.1.2/24, Area 0
Process ID 100, Router ID 192.168.1.65, Network Type POINT_TO_POINT, Cost: 1562
Transmit Delay is 1 sec, State POINT_TO_POINT,
Troubleshooting OSPF 163
OSPF routers exchange hello packets to create neighbor adjacencies. Four items in an OSPF hello
packet must match before an OSPF adjacency can occur:
■ Area ID
■ Hello and dead intervals
■ Authentication password
■ Stub area flag
To determine whether any of these hello packet options do not match, use the debug ip ospf adj
command. The output in Example 4-16 illustrates a successful adjacency on the serial 0/0/1
interface.
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