Line protocol goes up (after 'no shutdown), but immediately goes back down again

[TorontoCCNAStudent]

Would anyone know why, after pressing
int f0/0
no shutdown

and then receiving notice that the line protocol is up, it immediately goes back down again?

[Lion66six]

A few possible things for you.

If your using GNS3 to simulate this, it can happen that it will be in a flapping state. AKA it goes up and down several times a second. Are you using GNS3 or real equipment in this case?

If your using real equipment, then this is normally caused by a mismatch at the other end. For example what type of encapsulation are you using, and is there any authentication setup?

I would suggest doing the following command on both routers, and comparing them to ensure they work (From the privileged prompt) :

show interface FastEthernet 0/0

It should show a ton of data. Make sure encapsulation is setup the same on both, so default should be HDLC.

Does the command 'show ip interface brief' show any other details we can use?

[lammle]

If it's a FastEthernet, there is only a few possible answers, assuming you are plugging into a hub/switch from your router....

Bad interface on router
bad interface on switch or hub
bad cable or one of the ends of the cable is bad

That's why we use Ethernet, it isn't a great LAN technology, but's it's simply to troubleshoot.

Either way, sounds like a physical layer issue.

[Lion66six]

Yep I got this one wrong. Its a physical layer fault, not an encapsulation fault. In proving this to myself, I found it difficult in a simulation to change the encapsulation method on a FastEthernet link. I can use 'show interface FastEthernet 0/0' and see the encapsulation is set as ARPA.

Lammle, any ideas how to change this? A quick google search didnt show much, and I suspect its out of scope for CCNA?

[lammle]

We used to be able to change that encap on an Ethernet encap, but that stopped that more than a decade ago.
Ethernet_II (DARPA) is the default encap that cannot be changed.
That's what the whole world uses today...

[TorontoCCNAStudent]

Thanks Todd and Lion66six! I just tried the same with f0/1, and had the same result. I have my laptop hooked up to the router, and as these are the only 2 devices I have so far, I cannot unfortunately test the cables, because I have nothing else to hook them up to. However, if it's the same result on both the f0/0 and f0/1, I am assuming (and hoping) it's not the router. How can I test to see if it's my laptop?

[bs_kwaj]

I have my laptop hooked up to the router, . . .
You'll need a crossover cable for that connection.

Or, get one of these:

http://www.thinkgeek.com/gadgets/tools/7470/

And, one of these wouldn't be a bad idea either:

http://www.amazon.com/CCNA-640-802-Certified-Network-Associate/dp/0470901071/ref=sr_1_1?ie=UTF8&qid=1306934502&sr=8-1

:)

Or.... it may be the firewall on your laptop blocking the connection. ?? Maybe...

[TorontoCCNAStudent]

Thanks bs_kwaj! I've ordered the crossover adapter. The book I already have (and I had read the chapter, but, figured the advice I got on another forum on the type of adapter needed was good - I wish I had thought back to that chapter and checked the details:). Ah well, I have no IT experience and work in HR and therefore, all of this is completely new to me. The firewall settings I had already checked and updated, and so hopefully the cable is the only issue.

Thanks again!