WEBVTT

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 This video is part of the CCT routing
 and switching curriculum.

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 It comes from section 4 of that curriculum
 and the title is identifying

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 loopback plugs. My name is Keith Bogart
 and I will be your instructor

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 for this video. This video is
 going to be really short.

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 We're going to be focusing just on one
 bullet point in section 4 of that

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 blueprint, which is identifying the
 correct different loopback plugs.

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 That's all we're going to do,
 identify loopback plugs.

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 So, first of all,
 loopback testing.

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 Why would we do loopback testing?

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 What problem is solved?

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 Well, I think this is a pretty
 self-explanatory right here.

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 Sometimes when you're troubleshooting
 line errors, in other words, nothing's

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 getting through this port or what's
 getting through is all garbage or

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 maybe the port isn't even coming up,
 you might have in your mind, okay,

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 could it be one of
 these three things?

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 Is it possible there's a problem
 with my local NIC card itself?

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 Maybe the hardware of
 the NIC is damaged.

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 Maybe some of the pins
 are bent or broken.

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 Maybe the internal wiring has separated
 from some of the pins.

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 Maybe you have an internal memory issue
 or error, something like that.

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 So I want to test that.

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 How do I test that my local
 NIC card is actually okay?

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 Is there a problem with the
 cable connected to my NIC?

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 Maybe you've, so how
 do you test that?

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 Is the problem with the
 telco's line equipment?

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 Okay, maybe it's not my NIC card or
 my wire going to the wall jack, but

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 maybe it's from the wall
 jack to the telco.

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 Maybe it's something in their line or
 maybe it's something at their end.

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 So by doing loopback testing, it
 can help answer these problems.

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 Now what's loopback testing?

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 Loopback testing is real simple.

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 It really just involves, normally when
 you transmit something, it goes

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 out the transmit pin of your NIC card
 along the transmit wire strand within

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 that cable. And eventually it hits
 like a receive pin somewhere else.

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 Like a receive pin maybe at the wall
 jack where that cable is connected

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 to, there's a receive pin there.

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 And then it goes out the transmit pin
 and hits the receive pin at the

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 telco's location.

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 Or if we're talking about two switches
 connected back to back.

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 Same thing, transmit pin on the RJ45
 jack goes to the transmit wire, which

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 ends up in the receive wire here and
 the receive pin in this NIC card.

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 So loopback testing is simply saying,
 look, I'm not going to allow my

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 signal to go all the
 way from end to end.

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 Somewhere in that process,
 I'm going to loop it back.

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 I'm going to loop
 it back to myself.

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 And if that's successful, I know that
 from here to the point where I did

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 the loopback, at least
 that part is okay.

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 The problem must be somewhere
 beyond where I did my test.

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 So there's a couple of different
 kinds of loopback tests.

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 The one that we see here on the top
 is what's called a soft loopback.

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 Not all interface types
 support this.

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 But if you get into your interface, if
 it supports some kind of a loopback

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 command, for example, if you go into
 an ethernet interface and you type

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 in loopback, you should see
 that's a supported command.

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 What's that doing?

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 That's internally within the software
 that's saying, okay, when I'm sending

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 something this nitcard,
 send it back to myself.

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 So you're not changing the hardware
 characteristics at all.

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 It's what's called a
 software loopback.

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 You're getting the software
 to talk to itself.

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 And then there's other types of tests
 which are called hard loopbacks

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 and telcoassisted loopbacks.

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 Now, for the purposes of this video, because
 we're talking about ultimately,

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 what's our objective here?

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 The objective in that bullet point
 is to identify loopback plugs.

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 Well, a loopback plug
 is a physical device.

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 And all we need to
 do is identify it.

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 So we're going to be talking
 about hard loopbacks here.

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 Now, let me take a step
 back for a moment.

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 This video is for the CCT routing
 and switching exam.

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 Why is that important?

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 Well, because it says identify loopback
 plugs, notice what it doesn't

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 say. The bullet point doesn't say, understand
 how to use loopback plugs,

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 understand the correct test to
 implement with a loopback plug.

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 So I'm making the assumption here that
 because it's saying identify loopback

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 plug, you are like a field engineer
 or something and you're going out

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 to a site and you're on the phone with
 the Cisco TAC or some other kind

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 of engineer, it's the Cisco TAC's responsibility
 to figure out what types

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 of tests to do to do
 the troubleshooting.

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 So you're out on site and they're telling
 you something like, okay, we're

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 going to need to do
 a loopback test.

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 I need you to find an RJ45 Ethernet
 loopback or I need you to find a T1

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 loopback or a fiber loopback.

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 So it's just simply your job to look
 in your bag of all your equipment

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 and pull out a plug that matches what
 they're asking for and plug it in

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 the port. It's not your job to implement
 the test or to know what test

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 has to be run. That's
 why it says identify.

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 So with the last three or four slides
 here, I just want to show you what

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 these loopback plugs look like.

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 So if you ever see pictures of them
 or if you have a bag full of these

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 plugs, you can identify
 one from another.

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 You can differentiate
 them from another.

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 All right, the first kind.

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 What if we need to perform a loopback,
 a hard loopback test on an RJ45

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 port of an Ethernet Nikkart?

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 All right, well here you see on the
 left, this is what the pin out is

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 going to have to be.

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 So you can see pin one is going to be
 loopback to pin three, pin two is

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 going to be loopback to pin six.

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 And so your actual plug will look
 somewhat like on the right.

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 You want to make sure that the pin
 outs, that the wires are correctly

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 looped back to each other
 like you see here.

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 So that's what an RJ45 Ethernet
 loopback plug looks like.

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 Well, what if I have a fast or gigabit
 Ethernet interface that's doing

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 fiber? It's not doing RJ45.

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 Well, there are loopback
 plugs for that too.

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 You can see here the whole concept
 is going to be the same across the

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 board. Whatever's being transmitted
 is loopback to the receive and other

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 transmit pins have to be loopback
 to other receive pins.

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 So here we see a fiber
 loopback connector.

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 Main thing to remember here is that
 there's different kinds of fiber.

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 There's single mode fiber
 and multi mode fiber.

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 So ensure that whatever loopback plug
 you're selecting is appropriate

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 for the fiber interface.

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 So you should know, okay, this switch
 port or this router interface, which

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 is fiber, is supporting multi
 mode fiber, single mode fiber.

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 So I need to get a loopback
 plug that supports that.

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 There are some adapters that are both
 that support both single mode fiber

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 and multi mode fiber
 in one adapter.

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 So just make sure you
 do the correct type.

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 And the last one, actually
 no, there's two more.

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 What about a T1 loopback plug?

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 T1's typically plug into jacks that look
 very similar to RJ45, but they're

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 called RJ48C. So visually they look similar,
 but the pin out is different.

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 So you can see here on the left this for
 an RJ48C, this is what your loopback

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 pin out needs to look like.

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 And if this is of interest to you,
 you might want to pause this video

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 and write down that pin out and memorize
 that, you know, pin five goes

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 to pin two, pin one
 goes to pin four.

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 And then you see there on the right
 an example of what that type of plug

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 might look like.

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 Once again, a lot of these loopback
 plugs you can purchase, but a lot

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 of websites I've seen people
 have just made their own.

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 They just taken a cable, cut off the end,
 stripped some of the outer sheath

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 and just twisted the wires together
 themselves might just be cheaper and

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 easier for you to do it that way.

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 And the last type I want to talk about
 is an RS-232 serial cable loopback.

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 And as we know, RS-232 comes in
 many different form factors.

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 Here we see a couple of them.

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 So we see what the DB9
 loopback looks like.

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 And we see what the DB25
 loopback looks like.

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 Main point, main takeaway from all
 this stuff is to memorize what pin

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 is connected to what other pin.

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 Because sometimes when you're visually
 looking at the plug, you won't

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 be able to tell.

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 There might not be any indicator on there
 that's a loopback plug and you'll

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 have to take something off and actually
 look at how the wires are connected

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 to know if it is truly
 indeed a loopback plug.

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 And for the purposes of the exam, they'll
 probably want you to know that

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 this pin connects to this pin and this
 pin connects to this pin if we're

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 talking about T1 versus
 RJ45 on Ethernet.

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 So that concludes this video on
 identifying loopback plugs.