OSI Troubleshooting
Layer 3: routers

Routers are internetworking devices that operate at OSI Layer 3, the network layer. Routers tie together, or interconnect, network segments or entire networks. They pass data packets between networks based on Layer 3 information.

Routers make logical decisions regarding the best path for the delivery of data on an Internetwork based on Layer 3 information. Then routers direct packets to the appropriate output port and segment. Therefore, routing is sometimes referred to as Layer 3 switching. Routers come in a variety of sizes and form factors, but have common physical characteristics, such as LAN/WAN interfaces to provide connectivity between networks.

If IP or other services can be accessed on the LAN, but Internet access is not available, the router may be the failure point. Other connectivity issues, such as reaching other VLANs, can be attributed to a router. In many cases, the router is configured with access control lists (ACLs) to prevent unauthorized access. In a very secure network, adding new devices requires planning and coordination. Always consult the LAN/WAN administrator when connecting new devices to the LAN.

Routers provide numerous integrated commands to assist in monitoring and troubleshooting the internetwork, as shown in Figure . If no configuration problems exist on the router, other possible problems include cabling problems at the router or service provider outages.

Using show Commands
The show commands, shown in Figure , are powerful monitoring and troubleshooting tools. Use the show commands to perform a variety of functions, including the following:

  • Monitor router behavior during initial installation.
  • Monitor normal network operation.
  • Isolate problem interfaces, nodes, media, or applications.
  • Determine when a network is congested.
  • Determine the status of servers, clients, or other neighbors.