Windows 7 - Internetwork Troubleshooting Guide

Warning, This guide requires somewhat deeper knowledge in Internetworking terms.

Let's start...




To simplify things,...

What we have to remember here is that the OSI model is exactly that - a model. A physical piece of equipment can span more than one layer in the model, so a router can exist in both the physical and data link layers

The teaching of networking will be best left to the experts. I'm like yourself know what we know, from what we read here and there so if i am wrong, then i am wrong lol.

Ok, lets see how far i can go....

You wouldn't say a Layer 3 "device" broke plainly like that. Routers work off Layer 3 protocols. Each layer is connected to one another logically and information must travel through each layer down the order untill it reaches the physical layer where it then leaves your machine and heads towards the physical router. The information will go from the machine's interface port onto the medium (cable) to the router's interfact port. If say a ping of the router fails, the data packet will know this and it will either cease to exist or back track. Information then goes in reverse back to your machine, through the same interface, and up the layers. You won't know that a ping has failed untill the information reaches a layer that processes this information. Such activities when you break it down, might seem long, but in computer terms is can be as quick as a flash. When the machine gives you the info that a ping has failed, say host unreachable or request time out, you would then know that the ping has failed, and it failed not at physical level, but at a network level. Obviously if a ping fails at a network level (logical level) there is a good chance there might be something wrong with the router or an interface of the router, as the physical level.

Each machine work off the OSI model with protocols inter-linking from layer to layer on the machine. Information travels from Layer 7 down to Layer 2's logical link layer, and when it reahes the MAC layer of Layer 2, this is when logical becomes physical.

A router is a device/hardware seperate to the machine. Information goes from the machine to the router "physically" (thru a medium, and yes as hard as it may sounds wireless is also a form of physical connection) but the informatin of say a ping failure gets processed at the network layer. This is why the router is known to be a layer 3 device, as much as a hardware.

In my days in Cisco training lab (it was 2002/2003 iirc), when I encounter such problem I'm told to (according to my teacher's words) "trace from lower layers upward", this by his definition is make sure what you can touch, plug, etc is up and running, no diagnostic LED blinking, check cables, switch cables if you're in doubt, check that the plug is not loose, replug if loose, etc. This "trick" helped me in my 6 professional years as "All around network handyman".

I won't even start to guess "maybe it's driver, maybe it's X, maybe it's Y, maybe it's Z"... I'll start from the bottom (make sure the connectors are tight, LED indicator light up, etc), then go up (check ARP, ping, recheck ARP), then after that check the IP (ping), after that check the layer 4 (make sure that the ports are open and is listening to requests), and so on...

This is my "way" of doing things, I'm sorry if my way of thinking is not "in harmony" with yours .

zzz2496

The test case is all well except you might want to put in a Media Access Control Layer as part of the Data Link Later, at this half of Layer 2, it's where you talk about the adapters. The physical Layer 1, is where you say something like make sure you have the correct cable and it's inserted properly. It's up to you but for a non tech person that not necessary i don't think.

In my mind, when I experience ping failure, I instinctively will start from checking physical connections (in my word ==> "physical layer"), going up to second layer (MAC address, check this with ARP command), and so on... This way I know in what "layer" it fails and then act accordingly. I don't quite care about guessing why a ping command fails, it's irrelevant... I'll just trace it, by the time I arrive at layer 3(IP), most of the time I already know what the culprit was...

Note: In my days, I use many other link layer (layer 2) protocols other than Ethernet/MAC, I once setup Frame Relay connection (that's a Layer 2) using several Cisco routers over serial connection (thick heavy duty cables, Cisco proprietary), and it uses HDLC frames, at layer 3 uses IP. Another time I got a client that's running over microwave, I forgot what the Link layer protocol was... At that time, if I were think as if this network stack is one huge complicated mess, troubleshooting a failed ping will take days, even weeks... . That will definitely get my arse fired...

zzz2496

We are commenting each other's post in minutes intervals...

zzz2496

ZZZ, this is outstanding work!

After reading this guide, I suggest you read the other one...

zzz2496

I did! And is is excellent, too. I repped you on that one. Unfortunately, I can't rep you again.