The Winning that is Digital Optical Monitoring (DOM)

A number of Cisco Optical Interfaces implement Digital Optical Monitoring (DOM) which enables the monitoring of some interesting status values on the interface with the most useful values being the optical receive and transmit powers.

By being able to monitor transmit and receive power levels of optical interfaces you are able to characterise the fibre loss and isolate any unidirectional connectivity issues.

The following link is useful for identifying which optical interfaces support DOM – http://www.cisco.com/en/US/docs/interfaces_modules/transceiver_modules/compatibility/matrix/OL_8031.html

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Traceroute ain’t tracert

After ping, trace route is probably one of the most common network tools. Be it for fault finding, verification, discovery or testing. Between ping and trace route you can do a lot. Trace route, at it’s most basic sends a series of packets with an ever increasing Time to Live (TTL), starting at TTL=1. Every layer 3 device in the path will decrement this TTL and send a TTL Expired back towards the source if the TTL hits 0, until eventually the packet has a TTL that is long enough that it will reach the end device. The series of returned TTL Expired packets will tell you the path that the packet took.

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Bruce’s thoughts on Fault Finding

Fault finding is an incredibly subjective topic, I can’t think of the perfect fault finding process but there are a number of hints and best practices that a network engineer will pick up over time. From my recent TSHOOT studies, Cisco defines the fault finding process as:

  • Define the Problem
  • Gather Information
  • Analyse Information
  • Eliminate Possible Problem Causes
  • Formulate a Hypothesis
  • Test Hypothesis
  • Solve the Problem

This is a good position to start discussing my thoughts about fault finding issues in a network.

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