Troubleshooting: Common Layer 3 Problems

Here is a brief list of various issues that can impact layer 3 protocols in subtle or misleading ways:

  • MTU mismatch on a link
  • Unidirectional Traffic on a link
  • A duplex mismatch
  • High error rate in one or both directions
  • layer 2 configuration issues
  • ACL filtering with unintended consequences
  • Security policy blocking traffic
  • TTL setting is too low
  • more than one subnet in a VLAN

There are two basic approaches to troubleshooting:

  • Climb the Stack approach – begin at layer 1 and work your way up until you find a problem.  You can do it the other way around and start at layer 7 and work your way down until you find something wrong as well.
  • Divide and Conquer method – usually faster at finding the problem because you start at the layer thats having the issue.  You then either go up or down the OSI stack to find the issue based on the observed problems in the problem layer.

One thought on “Troubleshooting: Common Layer 3 Problems

  1. Thanks for this. If we look further down into each layer, then we need to understand packet headers to troubleshoot better. For example, we need to understand what packets are exchanged if the State machine is stuck at some point for a protocol. Agree with your basic approaches.

    Keep blogging Sir, do visit mine too. 🙂


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