Steps for Network Troubleshooting

And welcome back. Not every journey to

the world of troubleshooting ends the

same way. Some things are easier to

troubleshoot than others, and also, if you

and I have been working on, like, one large

network and we know it like the back of our

hand, it's a lot easier to troubleshoot

because we know what the subnets are and

the interfaces involved. Whereas, on the

other hand, if we go to a brand new

network or we're doing consulting, it may

take some warm-up time to get used to

and to figure out where everything is on

that specific customer's network. And when

we're doing troubleshooting--again,

whether it's our own network that we

know really well or it's a new network

that we are just introduced to--if we

have a certain process or methodology

for troubleshooting, we can apply that

methodology across the board. So let's

have some fun with this. We'll put an

overview of the high-level steps

regarding a troubleshooting methodology,

and then, as we proceed together, we'll

actually apply those steps as we

troubleshoot together a network. So the

very beginning of this troubleshooting

process would be to identify the

problem. Case in point: let's imagine that

the user who's sitting at this computer

right here, PC 10, calls the service desk

or the help desk, or they're

calling it in your organization, and they

say, "Yeah, I've got a problem." And then the

service desk says, "Okay, tell me more."

And if the user says, "Well, I can't really

tell you anything," well, we have to kind

of, you know, narrow down what the problem

is or at least get what the symptoms are.

And that's why one of the very first

steps is to identify the problem. So

with the identification of the problem,

the user may say, "I can't access the

Internet," or they may just say, "The

network is down." At which point, we would

ask some additional questions. So let's

imagine this user says, "I can't

access anything on the Internet." That

would fall into this category of

identifying the problem: this

user, who normally can access the

Internet, can no longer access the

Internet. The second step would be to

establish a theory regarding why that

might be happening. And so, by leveraging

a topology like this, we could ask

ourselves a few questions. For example, is

this computer powered on? If the

computer is powered on, does it have an

IP address? And if the DHCP client did

get the right information regarding a

default gateway and the subnet and all

that good stuff? And then regarding this

port--is this port on the switch

associated with the right VLAN, which is

VLAN 10? And regarding the trunking,

is it going down from the access layer

switch to the core? Is trunking working,

and is VLAN 10 being allowed? And then,

from the default gateway's perspective

regarding VLAN 10--who's acting as the

default gateway? Is it core 1 or core 2?

Or are they using a First Hop

Redundancy Protocol? And if so, which one

of these two devices is acting as the

active device? And does that device

acting as the default gateway have a

route out towards the Internet? In simple

terms, does it know how to forward? And

the same thing would hold true for this

router and then this connectivity to our

service provider. And also, because we're

using RFC 1918

addresses, perhaps network address

translation is failing or isn't

implemented correctly. So if this user at

PC 10, by doing a few tests, we verify that

it can ping its default gateway--And if

this device in VLAN 10 up here at

headquarters can ping devices out here

at Site 2 and Site 3 and has

reachability there, that can help

identify what is working, and then we

can establish a theory about what may

be specifically causing the problem. And

then, once we've narrowed it down to what

we think it might be, the third

step is to test, which is to basically go

in and prove your theory. If we think the

problem is with router one, or if we

think the problem is with a

multilayer switch, or we think the

problem is with the access layer, we want

to do some testing to validate that what

we think may be the problem really is

causing the problem. And then, once we've

narrowed it down and verified it, we then

want to go ahead and solve the problem.

Now, solving the problem in an

organization also has many steps

involved with it. Let's list a few of

those as far as the solution to this

network connectivity problem that the

user is having out to the Internet. And

let's also imagine, based on our testing,

that we believe it's an issue with

address translation, which could be NAT

or PAT, but definitely needs to happen at

some point before that traffic goes out

to the Internet. So if we've done some

testing and we've narrowed it down that

it is an address translation issue,

regarding solving that, we want to

create a game plan on exactly how we are

going to solve that problem. Perhaps with

network address translation, the NAT device

was set up to support VLAN 20 with the

10.12 subnet and other networks like

this over here at Site 2 and Site 3,

but maybe perhaps not including the

10.110 subnet. So we'd want to make a plan

to correct that. And also, in corporations,

that's going to involve going through

change control if we're going to make a

configuration change. And then, with the

authorization from the change control

board, we're going to go ahead and

implement the change. And then, when we've

implemented it, we also want to verify

that it's working. And that verification

would involve a few things: number one,

that we now have connectivity from this

PC up to the Internet. Also, we'd want to

verify that we didn't make any other

changes that would negatively impact our

environment. Like, we want to make sure that

everything else still functions as well--

VLAN 20 and the other sites--everybody can

still forward out to the Internet. And

then we'd also want to make sure we

document the solution--what we did, how we

did it. And if we changed the topology in

some fashion, we'd want to include that

update in our documentation. So the

documentation of what was done and also

the topology if there's been updates--

that's super important because, let's say,

3 or 4 days go by and we have yet

another problem. And we think, "Oh, I wonder

if what we changed here injected

additional problems into the network." So

we could go back through our paper trail

and identify what happened, when it

happened, what was changed. That can

help speed up our troubleshooting

because a lot of times, there are

cabling issues and physical issues and

so forth, but a lot of times when

something breaks on the network--when

something stops working--it's quite often

due to the last change that was made.

So if we go back and take a look at the

last change or two, that can help us

reduce our troubleshooting time by

either confirming that what was done is

not impacting our current problem or by

verifying that what was done indeed is

impacting our current network. And then

the last step here is to go ahead and

repeat this process for the next problem.

So the next service call that

comes in, the next issue, the next problem--

again, we're going to follow this logical

plan. So what I think would be fun to do

is let's take this network topology,

which we've been playing on and off with

throughout these videos, and what I'll do

is I will inject a problem somewhere in

this mix, and then we can go through

these steps one at a time in this

troubleshooting methodology. And as we do

so, we'll go into more details on each

one. So, in the very next video, join me as

we take a look at this first stage in

the troubleshooting methodology, and that

is identifying the problem, which we'll

do in this network topology. So I'll see

you in that video in just a moment.

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