Knowing the Interfaces

DANMOH009

This question has been bugging me for a while, I mostly use packet tracer to configure networks but from time to time i forget which interface is which.


Is there a way of checking an interfaces on a router in CLI if there is just a cable connected to it with no configuration? there will be times when i connect up a cable to a FastEthernet or Serial Port, and i forget which interfaces i connected it to, I normally have to go to show port labels in the preferences tab of packet tracer.

I was wondering is there a command that will show this, bearing in mind no configuration would be set on the interface just a cable ill be connected to it

Sorry in advance if this is a stupid question.

sratakhin

show ip interfaces brief
or shorter,
​sh ip int br

inscom.brigade

This is something that we will do all the time

"Show commands"

show interface ?
show interface fasterthernet (0/1, or any number)
show cdp neighbor

You must consider the importance of an up to date blueprint diagram, (we call that the topology).


show ip interface brief
show vlan
show ip route

DANMOH009

Thanks guys,

I kinda think im not getting understood though, I mean i know the basic show commands. But lets say i i connect 2 cables up to a switch or router and dont configure anything. Then i go start configuring something else.

When i come back to the device ive plugged the cables into, if nothing is configured on them, i log into the CLI of the router, how do i remeber which interfaces the cables are plugged into?

A show ip int brief wont show anything, neither will a show interfaces.

I just wondered if there was another command,

boredgamelad

DANMOH009 wrote: »

When i come back to the device ive plugged the cables into, if nothing is configured on them, i log into the CLI of the router, how do i remeber which interfaces the cables are plugged into?

The short answer is, you write it down.

Proper network documentation = happy network engineer.

chmorin

If you run "Show IP Interfaces Brief" you get a list of the interfaces in the router, their IP Addres (if any) and physical & logcial connectivity status.

Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0          unassigned      YES unset  administratively down down

You have a few options.

Status: Up, Down, or Administratively Disabled.
Up means the interface is plugged in and 'lit'.
Down mean that there is no physical connection detected.
Administratively Disabled means the interface has likely been put in the 'Shutdown' state.

Protocol: Up or Down.
Up means that there is some amount of Layer 2 or 3 or w/e connectivity on the interface.
Down means that... there isn't.

That being said, with no config, its likely everything will show up as "Administratively down" on a router. So you can't really tell from a fresh config.

inscom.brigade

You can figure it out without getting up to look.
It will take longer but you can succeed.
We will assume that we are talking about a CCNA level lab topology not a CCIE enterprise, campus lab rack.

So with that said we are safe to say that we will be talking about only a few routers and a few switches.
From a simple Show ip int br command: we will see our interface list. Since it is new all will be administratively down. We will have 1, or 2 ethernet interfaces, or fastethernet depending on model router. If we issue a no shut on each interface, then issue a show ip int br command, we wil then see up up, or up down.

Musixa

DANMOH009 wrote: »

Thanks guys,

I kinda think im not getting understood though, I mean i know the basic show commands. But lets say i i connect 2 cables up to a switch or router and dont configure anything. Then i go start configuring something else.

When i come back to the device ive plugged the cables into, if nothing is configured on them, i log into the CLI of the router, how do i remeber which interfaces the cables are plugged into?

A show ip int brief wont show anything, neither will a show interfaces.

I just wondered if there was another command,

If I understood you correctly, you might be asking how to remember which interface of the device, you are about to configure, to which another device's interface that it is connected? For example, R1 (fa0/0) connected to S1 (fa 0/0), and you want to find out what R1's fa 0/0 connected with using a show command? If this is your question, then you must use the show cdp neighbors detail command. This would tell you what device is connected to your interfaces. It would let you see the details of the neighbor's device and would let you know that neighbor device your interfaces connected to.

boredgamelad

I don't think anybody understands what he's getting at. Everyone seems to be making the scenario more complicated than the simple one provided.

He has a brand new router, R1. He plugs in an ethernet cable to fa0/1.

He has another brand new route, R2. He plugs the other end of the ethernet cable to fa0/0 on R2.

He goes to lunch. He comes back, and sits down at the console for R1. He forgets which interface he plugged the cable into. None of the interfaces have configurations.

His question is: is there a way to tell if any interfaces on R1 have cables plugged in, without setting any configs, and without physically checking the router?

Note: He's not trying to figure out what's connected to R1. He just wants to know if any cables are connected to the interfaces.

mohamedshajid

boredgamelad wrote: »

I don't think anybody understands what he's getting at. Everyone seems to be making the scenario more complicated than the simple one provided.

He has a brand new router, R1. He plugs in an ethernet cable to fa0/1.

He has another brand new route, R2. He plugs the other end of the ethernet cable to fa0/0 on R2.

He goes to lunch. He comes back, and sits down at the console for R1. He forgets which interface he plugged the cable into. None of the interfaces have configurations.

His question is: is there a way to tell if any interfaces on R1 have cables plugged in, without setting any configs, and without physically checking the router?

Note: He's not trying to figure out what's connected to R1. He just wants to know if any cables are connected to the interfaces.

Yes of course this is the right answer to realize what's the situation is it? Even you can use Show Interface i think.

SteveO86

Let's not forget about good old interface descriptions, and put descriptions on your important interfaces.

sratakhin

Can't you just look at the routers and see where the cables are plugged in?

RouteMyPacket

boredgamelad wrote: »

The short answer is, you write it down.

Proper network documentation = happy network engineer.

This!

Anytime you set an interface, do a "description ** Trunk - C3560_SW1 G0/1 **

Something like that

chmorin

Except that's a configuration of the interface...

This is a silly question.

DANMOH009

DANMOH009 wrote: »

Sorry in advance if this is a stupid question.

I did say sorry if it was a stupid question. Cheers anyway guys

NotHackingYou

DANMOH009 wrote: »

I did say sorry if it was a stupid question. Cheers anyway guys

It's not a silly question if it gets people thinking. Good question!

boredgamelad

It is kind of a silly question, but it has made for some interesting answers.

Show interface won't help--two ethernet interfaces with no config look exactly the same regardless of whether a cable is plugged in. On an ethernet interface anyway, even after issuing a no shut, they will show the same (note: some research has led me to believe this doesn't hold true for all IOS versions). Serial interfaces are another story--if you issue a no shut, you should get up/down if a cable is plugged in and down/down if there's no cable, even without any other config.

Now, if you're coming into a network that has been poorly documented, then maybe this is less of a silly question and yeah it would be nice to have a command that you can run to see what's cabled and what isn't. But, even then, you're never gonna really know what's going on unless you can get a good physical look at the router.

I stand by my previous post re: documentation and good network design. All you can do is design and document your network well and hope others do the same to save you trouble in the future.