Routing Table Issues

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• if you can ping a host’s default gateway, but cannot ping some or all hosts on remote networks,
  • then suspect a routing issue
  • many cases this will be because a router has gone offline and there is no alt path to the network
  • verify:
    • the router is powered on
    • cabling to interface ports is correct
    • each interface is up and configured with the correct IP address
  • if all is good, then investigate the routing topology
• if suspect problem with router configuration and network topology:
  • use traceroute to identify where the network path is failing
  • use route or show to investigate the routing tables of intermediate systems at that point in the path
  • when inspecting a routing table:
    • use show ip route w.x.y.z to check for the presence of a route to a specific IP network
      • a missing route may arise because:
        • a required static routing entry has not been entered or has been entered incorrectly
        • a router fails to communicate with its neighbors and does not receive routing protocol updates
      • troubleshoot by:
        • pinging the router nodes that are neighbors of the system with the issue to check basic connectivity
        • if there is a network path and neighbors are up,
          • investigate the protocol configuration
            • could be authentication issue or incorrect parameter
• If all expected routes are present, may be a priority problem
  • route selection uses the following factors:
    1. most specific path is preferred
    2. if paths with equal prefixes, the path with the lowest administrative distance will be selected
       • administrative distance is a measure of how trustworthy the source of the root is
       • directly connect and static routes have lower AD values than routing protocols
    3. if identical paths with equal AD, the path with the lowest metric value is preferred
  • investigate any paths with overlapping ranges
    • i.e., /24 and /28
    • likely to indicate an error, especially if from different sources
    • e.g., a misconfigured static route might be in conflict with a learned route