Network layer problems include any problem that involves a Layer 3 protocol, both routed protocols (such as IPv4 or IPv6) and routing protocols (such as EIGRP, OSPF, etc.).
Common symptoms of network problems at the network layer include:
- Network failure - Network failure is when the network is nearly or completely non-functional, affecting all users and applications on the network. These failures are usually noticed quickly by users and network administrators, and are obviously critical to the productivity of a company.
- Suboptimal performance - Network optimization problems usually involve a subset of users, applications, destinations, or a particular type of traffic. Optimization issues can be difficult to detect and even harder to isolate and diagnose. This is because they usually involve multiple layers, or even the host computer itself. Determining that the problem is a network layer problem can take time.
In most networks, static routes are used in combination with dynamic routing protocols. Improper configuration of static routes can lead to less than optimal routing. In some cases, improperly configured static routes can create routing loops which make parts of the network unreachable.
Troubleshooting dynamic routing protocols requires a thorough understanding of how the specific routing protocol functions. Some problems are common to all routing protocols, while other problems are particular to the individual routing protocol.
There is no single template for solving Layer 3 problems. Routing problems are solved with a methodical process, using a series of commands to isolate and diagnose the problem.
Here are some areas to explore when diagnosing a possible problem involving routing protocols:
- General network issues - Often a change in the topology, such as a down link, may have effects on other areas of the network that might not be obvious at the time. This may include the installation of new routes, static or dynamic, or removal of other routes. Determine whether anything in the network has recently changed, and if there is anyone currently working on the network infrastructure.
- Connectivity issues - Check for any equipment and connectivity problems, including power problems such as outages and environmental problems (for example, overheating). Also check for Layer 1 problems, such as cabling problems, bad ports, and ISP problems.
- Neighbor issues - If the routing protocol establishes an adjacency with a neighbor, check to see if there are any problems with the routers forming neighbor adjacencies.
- Topology database - If the routing protocol uses a topology table or database, check the table for anything unexpected, such as missing entries or unexpected entries.
- Routing table - Check the routing table for anything unexpected, such as missing routes or unexpected routes. Use debug commands to view routing updates and routing table maintenance.