Common OSPF problems and its troubleshooting

Certification: Cisco CCIE Routing and Switching - Cisco Certified Internetwork Expert Routing and Switching

OSPF is the abbreviation of Open Shortest Path First. The basic purpose performed by OSPF is to find the shortest path to a destination on a network to make sure that the element of cost in the process in minimum. OSPF is basically a link-state routing protocol which is most commonly used nowadays. It also comes under the category of “open” protocols. It uses a very intelligent way to carry out its purpose. It gathers the information from all the routers in the surrounding areas to make sure that data chooses the smallest path to be transmitted from one place to the other. It can effectively manage the bandwidth of the network and can easily operate on complex networks. This amazing quality of OSPF has made it one of the most important components of a network. However, there are certain problems which have been encountered with OSPF and which pose a threat to the data packets which might not be able to take the shortest path due to these problems. Hence, it is very important to identify these problems and devise suitable ways to troubleshoot them. Following are the most common issues faced with OSPF and ways in which they can be troubleshoot.

OSPF Neighbor Issues

You should be aware of the fact that OSPF calculates the shortest path for data by getting information from the nearby routers to figure out which path is the shortest for the data to travel. This connection with the neighboring devices is very important as the data is transmitted due to the information sent from one router to the other. However, if there is some problem with the connection between two devices, OSPF will not be able to identify the shortest path. This could lead to delays in the transfer of data and reduced speed of network.

In order to troubleshoot this problem, you need to make sure that all of the requirements which are responsible for the establishment of a connection between two routers are met. Firstly, you must make sure that both of the OSPF devices such as routers are on the same subnet. This could be done by checking their subnet mask to see if it is same for both. In addition to this, both of these devices are supposed to be in the same area so they are able to make a connection. Most importantly, you should check whether these OSPF devices have been provided their unique Router-IDs. These IDs are important to identify each router separately in a network. Once of all these conditions have been checked and resolved, OSPF will start to function again normally.

OSPF Routing Table Issues

OSPF makes use of routing tables to identify the shortest distance for data to travel. The routing table comprises of all the information regarding the position of each router, distance between each router and sometimes the direction of each router from the other one as well. OSPF makes use of this information to calculate the shortest route for data to reach its destination. However, sometimes the routing details from the routing table are deleted. These details can relate to the external as well as the internal routes. Under such conditions, the OSPF is not able to function properly.

In order to eliminate this issue, one first needs to identify which routes have been deleted or have been damaged in the routing table. If all of the OSPF routes have been deleted from the routing table, the problem is quite crucial. In this case, you will have to carry out a full adjacency check. If only the external routes have been deleted, routes which have been originated from another process during routing, you need to carry out an external LSA check. If the summary routes, routes which originate from another area, or NSSA routes have been deleted, you should open the TAC case to resolve this issue. It is crucial that the routing tables are repaired in the shortest possible time so that the network could get back to normal and OSPF could start working properly again.

OSPF INIT state issue

INIT state means that one of the router is able to send OSPF “hellos” to the neighboring router, but the neighboring router is not able to send back “hellos”. This could lead to the problem of communication between the two routers which would prevent OSPF from performing its task. If this problem arises, you first need to check if the OSPF authentication is being used in both of these devices. You will have to make use of the “show IP OSPF interface” command in order to check this. If you figure out that the same authentication is being used, then you should check whether the same authentication keys are used by both of these devices. However, if you find out that the authentication type on both of these devices is not same, you should check whether the physical cabling has been done properly. In this regard, you should also check if the switch settings have been done properly. If you find issues in any of these steps, you should immediately resolve them as mentioned.

ACL Issues

The primary purpose of Access Control List (ACL) is to filter the data when it passes through a router. However, the process of ACL interferes with OSPF and prevents it from working properly. Hence, you need to check whether any of the routers are configured with ACLs. You could check this by using the command “show ip interface”. If you find out that if ACL is configured in any of the devices or routers, you should immediately disable the ACL to check if the OSPF starts working fine again. If it does, then you could reconfigure the ACL settings to ensure that it does not interfere with OSPF anymore.

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