When configuring routing on network devices, you have to carefully configure static or dynamic routing based on the design. If you have a problem and are not able to send/receive traffic across the network, you probably have some kind of configuration issue.
When you initially set up a router, there will most likely be some type of configuration problem that you will have to troubleshoot. If the router has been running for a while and there is suddenly a complete failure of traffic (i.e., no communication), you should analyze the situation and figure out whether the routing protocol is functioning as expected.
Sometimes certain routes might intermittently disappear from and appear in the routing table, causing intermittent connectivity to specific destinations. This may be because a certain network area has some kind of communication problem and routers along the path propagate new routing information every time that area becomes accessible. This process is called route flapping and specific routes can be blocked using a feature called route dampening so that they do not affect the entire network.
| Note: When using static routing, the routing table never changes so you will never know about problems that occur in different network areas.
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When troubleshooting routing issues, the standard approach is to follow the routing table for every route along the path. You might want to perform a traceroute to understand exactly where the packets are going and to see routers along the path. This should point to the devices that are causing the issues, so you can start investigating the routing tables on those specific routers.
A common mistake when performing this kind of troubleshooting process is investigating the issue in a single direction (e.g., source to destination). Instead, you should perform the troubleshooting process in both directions because you might come across situations in which packets are blocked in a single direction so there is no return traffic from the destination to the source. The routing tables on devices along the path between two points should correctly point in both directions to ensure an optimal traffic flow.
Often you will be using connections provided by third parties, so if you want to troubleshoot issues in that area, you should communicate with the service provider and synchronize the troubleshooting effort. This includes sharing routing table information.
Using dynamic routing protocols makes the troubleshooting process easier because you can inspect the routing updates as they are sent and received by the router. This can be done via packet captures or internal device mechanisms, which can point out how and when the routing table is populated. Having a topology map and other documentation that lists where every prefix is located in the network can further help your understanding of the routing updates and can shorten the troubleshooting process.
The general idea in such a troubleshooting process is deciding which path a specific packet should take, based on the network design, and investigating where exactly it is deviating from this path.
Different tools can be used to monitor network devices. A common network management protocol used by these tools is Simple Network Management Protocol (SNMP), which was designed to query network devices for different parameters from a management station. In addition to the standard “health” parameters checked (e.g., CPU, memory, disk space, etc.), SNMP can also query the router for things like:
- Interface packet counters
- Bandwidth used and throughput
- CRC or other types of errors on device interfaces
- Routing table information
Other types of tools that you can use are standard ping and traceroute utilities to verify end-to-end connectivity. They can also show relevant output that might help determine the point in the network where problems are occurring.
By properly configuring the routers in your network, documenting each step along the way, and constantly monitoring the path between different points in the network, you should have a solid understanding of exactly how the traffic should traverse all the devices in the network.