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It just says, hey we're not getting there and you're going to take my word for, and actually we don't have to take my word first, give it a three. Now I know you're thinking why are you pinging two from three when you couldn't being three from two.
And in a smaller lab like we're working with here, I grant you it probably wouldn't work but in rare occasions and more advanced labs especially if you've got your eye on the IE, you'll see scenarios very rare but they do happen where A can't ping B but B can ping A. That obviously is not the issue here though, so we know they can't talk to each other.
What are some basic commands here that'll help us get the lay of the land because what we have here is a diagram with a couple of IP addresses and you know it never hurts to check those because I'm going to paraphrase a guy I talked to many years ago.
The network diagram ain't always right. And it's not, you kind of looked at ones that had incorrect numbers on them, so it doesn't hurt to check that and we doesn't hurt to check the state of the interfaces either because if the interface is shut down, what state is it in, it's an administratively down state. First thing you learn an IP routing, right? So let's go ahead and take a look at the interfaces in question. And we're really just concentrating on the top lines here and there's your MAC address, your IP address, it has an IP address so you're good there.
As a refresher, as to what exactly that means, the first part of it refers to the physical state of the interface, the second part to the logical state. So usually with an Ethernet interface, Ethernet based interface I should say, you're going to see either up and up or down and down, you can't see the physical interface up while the logical state is down but that happens more on serial interfaces when you're working with different end cap types like frame relay and PPP.
So everything looks pretty good over here on a router three, let's take a look at router two. And again this is just a refresher, just to remind you to look at the simpler things first, make sure this baby's got an IP address, make sure it's up and up, that kind of thing before we start getting into some more complex solutions. And everything actually looks pretty darn good there.
So how can I make absolutely sure and this is something you really want to check laughs. With some network maps out there in the real world, they're pretty bad about those. Let's say you were told, oh well router two is connected to port fast 02 on the switch and router three is connected to fast O3 on the switch.
Let's say I told you that verbally, well I just did, but let's say that it was actually on the diagram but you wanted to verify it because as veterans and my videos are about say we don't mind trusting but we always verify.
How can I make absolutely sure that the physical connections are what I think they are or what I'm being told they are? And that's where CDP comes in. You can't run it on non cisco devices but it's really helpful in making sure that your cables are where you think they're supposed to be. So if you were just making yourself a little network diagram either in the exam room or in the real world, this is how you do it.
CDP is a great help as long as you have all cisco devices of course. And then I'm going to run the exact same thing over on three and just make sure my cables are where they're supposed to be and that they are connected of course. Now, we ought to go check things on the switch and just make sure everything's what we think it should be there and we'll run show CDP neighbor. And of course what I want you to get used to here is seeing other devices because of course in an exam room or certainly in a production network, it's not like you're just going to see the one port you need information on right, you're going to see all of the CDP.
So it would appear as though switch one is connected to a switch two, I'll slide that over just a bit there, and you can see it's connected to switch two via ports 11 and And we don't expect to see an IP address 'cause we're running this as a layer two switch but we see up and up and even connected in paren there in the first line of the output.
So, so far so good, won't hurt to check three and we're up and up there so everything's good. So what else could be causing an issue here, what else would I check on a layer two switch. I've checked all the physical connectivity, I'm good at layer one. What other layer two feature might we have on a switch here that could be causing this kind of connectivity as your issue? And this is the full show VLAN and I know the lines hitting it right there, let's move that up, and you can see that we have a whole mess of interfaces in there and VLAN one which is of course our default.
And the only way we can have inter VLAN traffic is what? What do we have to get involved? We have to get layer three involved because layer two switches are not going to be able to handle inter VLAN traffic. When a switch receives a signal, it creates a frame out of the bits that were from that signal. With this process, it gains access and reads the destination address, after which it forwards that frame to the appropriate port. Now We will discuss the topics mentioned above briefly to have a bit more clarity and understanding of the subject.
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