Allied Telesis AT-8900 SERIES Betriebsanleitung

C613-16028-00 REV B www.alliedtelesis.com

Best Practice |

Introduction

This How To Note describes techniques for on-site debugging. It is in no way intended to be

a comprehensive guide to debugging Allied Telesis equipment. Instead, it is aimed at passing

on some techniques that have been found to be useful. Particularly, this document describes

techniques which:

zmake it less likely that you will miss important pieces of information

zmake it simpler for someone else to analyse the debug information that you capture

zenable you to be more accurate in after-the-event discussions of what you saw.

Which products does it apply to?

This document applies to the following Allied Telesis routers and managed layer 3 switches:

zAR300, AR400, and AR700 series routers

zAT-8600, AT-8700XL, AT-8800, Rapier, and Rapier i series switches

zAT-9800 series switches

zAT-8900, AT-9900, and AT-9900s series switches

zx900 series switches

zSwitchBlade series switches

Related How To Notes

You also may find the following How To Notes useful, because they specifically discuss how to

debug connections:

zHow To Troubleshoot ISDN Connections

zHow To Troubleshoot VPNs

Note that many other How To Notes include troubleshooting information as well as

configuration examples.

How To Notes are available from www.alliedtelesis.com/resources/literature/howto.aspx.

On-Site Debugging

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:

za cable plugged into the wrong socket

za misconfiguration of some piece of equipment

zan interoperability problem between two pieces of equipment

za hardware fault in some piece of equipment

za 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

show ip count

show pim route

show pim count

show pim neighbour

and other commands relevant to the hardware forwarding of multicast in the particular

models of switch you are working with.

Once this initial survey is complete, you can start following up your theories about what is

going on.

Following up theories

Starting from the symptoms of problem, you can work back through the steps in the

networking process to think of various points where something could have gone wrong, and

follow up on each of those in turn.

For example, if the symptom is that a certain client never receives multicast streams, then the

problem could be:

zthe client is failing to send the IGMP report; or

zthe client is sending the report, but it is being dropped by a switch between the client and

the device running PIM; or

zthe IGMP report is reaching the PIM router, but a multicast route is not being created;

or….

So, the approach is to follow each of these possibilities—capturing counters, enable debug

outputs, ethereal traces, etc to confirm or deny each one. When a particular possibility looks

fruitful, then dig deeper into it to try and narrow down more accurately what is not

happening correctly. For example, if you see that the IGMP report is not being seen by a

particular switch in the chain, you might be able to find that an error counter in the show

switch port count output increments every time you try sending the IGMP report,

indicating that the reports are being corrupted.

It is important to always keep in mind that it is quite likely that your debug capture will be

analysed by someone else later on (or, in fact, you might need to go back through it later

yourself). Therefore, it is necessary to record your thinking and actions as you were going

along, to give context to all the debugging that was captured.

This achieves two main aims:

zit makes it clear why you were looking at particular counters etc

zit makes it clear how the captures relate to external events. So often it is vital to know

whether a particular state change or counter change seen in the debug capture happened

before or after a particular external event. For example, 'did that switch port on blade 6

go into STP blocking state before or after port 24 on blade 5 was unplugged..'.

Concrete piece of advice #3: Type your thoughts and actions.

All the way through the capture, type (just at the CLI prompt) what you are thinking, and

what you are doing. Just write it as stream-of-consciousness, things like:

“Hmmm.. that counter value looks strange, lets look at this a bit more”

“I have just pulled out blade 5, lets look the internal ports…”

“Right, the ping to the PC on port 5 just starting failing “.

Of course, your consciousness might not stream in English. If so, then write the comments in

your language; the important ones can always be translated later. The important thing is that

the writing of the comments does not get in the way of the job at hand.

Of course, writing comments at the CLI prompt will mean that you get error messages from

the unit like:

Manager > Now I am going to send the IGMP report from the client,

and see if any PIM counters or states change

Error (3035256): Unknown command "Now".

But that is quite OK, the person analysing the capture will be able to mentally filter out these

error messages.

When you see something really significant, you might want to highlight it, so it is easy to find

later. A string of !!!!!!!! or ###### are good ways to make sure something stands out when

you are browsing through the file.

What counters are changing?

Often, the investigation of a theory requires looking for what is changing—for example, are

the error counters on a particular port increasing? Is Port A now sending more multicast

packets than Port B is receiving? Are entries in the route ARP table being refreshed? This

brings us to concrete piece of advice #4.

You might see certain values changing that give a vital clue to what is happening. Or, if you

don't notice them, the person analysing the capture after you might.

Rate of change can be important

More subtly, the rate at which certain things change can often be significant—for example,

how long did it take that unit to make the transition from VRRP master to slave? or how long

did that route take to age out? or other such questions. Unfortunately, the outputs of show

commands are not time-stamped, so sometimes it is necessary to do the “time stamping”

yourself:

Concrete piece of advice #4: Perform particular show commands a few

times in succession.

Concrete piece of advice #5: If you are investigating matters that are

time-related, then enter a “show time” command before each of your

other commands.

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C613-16028-00 REV B

Providing a convincing case

If you believe that you have successfully hit on the right theory, and have isolated what

appears to be a good candidate for the root cause of what is going wrong, gather as much

evidence as you can to support the theory.

Often certain pre-conditions are pertinent to the circumstances in which the problem will or

won't occur—'the problem still occurs even if port 2 is not connected'; 'if the ARP entry has

been cleared before the route ages out, the problem does not happen' etc. It is important to

capture the output to illustrate these pre-conditions—it might be very clear to you that port

2 is definitely disconnected, but the person analysing the captures after you might not truly

believe that unless they see the output of the show port command that shows port 2 to be

DOWN.

Annotating afterwards

Before you send your capture off to the next layer of support or anyone else, you will want

to give them as much guidance as possible as to how to extract the significant pieces of

information from your capture. This can be achieved by going through the capture and

putting in further comments to spell out more clearly what is happening than you were able

to do with the quick comments you typed in while on-site. Or, you might want to copy the

really important parts out of the capture and paste them into a separate file, to succinctly

illustrate the core of the problem.

But, even if you copy the pertinent pieces out of the capture, still send the rest of the capture

file anyway, there could be significant gems of information in there that you weren't aware of.

Concrete piece of advice #6: If you find a sequence of actions that make

the problem happen, capture the full sequence (and the evidence that

the problem has happened) multiple (more than 2) times if possible.

Concrete piece of advice #7: Capture as much evidence as possible to

support any assertions you make about the circumstances in which you

see the problem happen.

Concrete piece of advice #8: Some post-processing of the capture file

can save a lot of time and effort for anyone who is analysing it after you.

Concrete piece of advice #9: If possible, always send everything that

you captured—too much information is always better than too little.

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