OSPF is one of the more complicated routing protocols, and it can be pretty threatening. Therefore troubleshooting OSPF and verify the configurations are very important. There are several ways of verification and troubleshooting OSPF:-
Verify OSPF Neighbors
Figure 1 shows the reference topology for this lesson. We can show and verify the OSPF neighbor adjacency using the “show ip ospf neighbor” command. If the router not showing the state of Full, the routers have not formed an OSPF adjacency.
The causes of routers do not establish adjacency, LSAs not exchanged between routers. Incomplete LSDBs also can cause inaccurate SPF trees and routing tables. If the route to the destination does not exist, or also may not be the most optimum path for the destination. Figure-2 displays the Router1 neighbor adjacency.
The parameter shown in this command are as under:-
Neighbor ID – The unique ID of the neighboring router.
Pri – The OSPF priority of the interface. This value is used in the DR and BDR election.
State – This is the OSPF state of the interfaces. FULL state means that the router and its neighbor have identical OSPF LSDBs. The dash indicates that no DR or BDR is required because of the network type. On multi-access networks, such as Ethernet, two adjacent routers may have their states displayed as 2WAY. The DR show the adjacent router connected to this interface is DR.
Dead Time – The remaining time to receive an OSPF Hello packet from the neighbor before declaring the neighbor down. This value is reset when the interface receives a Hello packet.
Address – The IP address of the neighbor router interface to which this router is directly connected.
Interface – The router interface on which it has formed an adjacency with the neighbor router.
We can also verify Router2, Router3 and Router4 using the “show ip ospf neighbor” command and get help troubleshooting OSPF on these routers. If the adjacency did not form any router. Check the following:-
If the subnet masks do not match.
OSPF Hello and Dead timers do not match.
OSPF Network Types do not match.
There is a missing “network” command
Incorrect OSPF “network” command.
Verify OSPF Protocol Settings
We can verify the OSPF protocol settings using the “show ip protocols” to verify important OSPF configuration information. The command displays the OSPF process ID, the router ID, the advertised network, the neighbors the router is receiving updates from, and also the default administrative distance. The figure-3 Illustrates the output of the “show ip protocols” command. Use the command on other routers to verify the above information.
Verify OSPF Process Information
We can also verify the OSPF process ID and router ID using the “show ip ospf” command, as shown in Figure 4. This command displays the OSPF area information and the last time the SPF algorithm was calculated.
Verify OSPF Interface Settings
The command “show ip ospf interface” can display the interface settings. With this command, we can display and check a detailed list for every OSPF-enabled interface. The command of whether the network statements are correctly composed or not.
We can only get the summary using the command “show ip ospf interface brief”. Figure-5 display the detailed information of this command. We can also specify the interface name with this command to display the information of the particular interface for example “show ip ospf interface serial 0/3/1.
Verify OSPF Database
This command “show ip ospf database” displays the information on the number of routers in the network or internetwork plus the ID of the neighbouring router.
The figure-6 illustrates the output of this command. The ADV router is the advertising router. The checksum link count might display different numbers depending on the routing device.
The Debugging of OSPF
This is an important command for troubleshooting OSPF. We can use this command in the following ways:
Debug ip ospf event: This command displays hello packets sending and receiving.
Debug ip OSPF adj: shows DR and DBR elections on broadcast and non-broadcast multi-access networks.