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Starting off
When going on-site to investigate a network problem, your guiding aim is to understand the
root cause of the problem, which might be:
z a cable plugged into the wrong socket
z a misconfiguration of some piece of equipment
z an interoperability problem between two pieces of equipment
z a hardware fault in some piece of equipment
z a bug in the software of some piece of equipment.
Of course, it might not always be possible to find the root cause, but looking for it should be
the central target of your debugging session.
However, it is also important to make sure that you are also capturing enough information
(and the right sort of information) to enable someone else to effectively analyse what you
have captured. This could be so that they can confirm your theory about the root cause of
the problem, or, in the case that you were not able to nail down the root cause, so that they
can develop their own theories about what the cause might be.
Right from the first moment you attach your terminal emulator to the console port of the
first piece of equipment, ensure that the logging facility of the terminal emulator is turned on.
This means that you will capture a lot of (seemingly) irrelevant stuff, but it also means that
you will not fail to capture the important stuff.
It is important to approach the problem with an open mind, and start with a general survey
of the equipment you are looking at, to give an initial picture of how things were at the start
of debugging session. This might be the “network operating OK” state, or the “things going
wrong” state, depending on what state the network was in when you arrived.
In this initial survey, you should capture a show debug from every relevant unit, and capture
all the counters and module states that are relevant to the issue.
For example, if the network consists of switches, and the problem is related to PIM multicast
routing, then you will want to capture the following commands:
show switch count
show switch port=all
show switch port=all count
show ip route
show ip route multi
show ip igmp
Concrete piece of advice #1: Capture everything
Concrete piece of advice #2: Capture the initial state