"Line protocol is down" ?

persona

When issuing the command:
sh int s0 for instance, we may get th efollowing info:
Serial0 is up, line protocol is down.

Cisco says that this is a Layer 2 problem.
I thought since this is a Layer 3 problem (Router is lacking a routing protocol). Guess I was wrong.

Would someone clarify this matter to me? Thanks.

markzab

To quote...

"If you see that the line is up but the protocol is down, as shown above, you’re experiencing
a clocking (keepalive) or framing problem. Check the keepalives on both ends to make sure that
they match, that the clock rate is set, if needed, and that the encapsulation type is the same on
both ends. The output above would be considered a Data Link layer problem."

Make sure you have your clock rate set on your DCE. Also check up on your layer 2 encapsulation and make sure it is the same on both sides of your WAN connection.

Also, to answer your question about layer 3, this isn't actually a problem with routing at the 3rd layer. You may have a routing protocol set up on your machines but you have to makie sure that your framing protocal at layer 2 is matching on both sides as well. Frame Relay, HDLC, PPP, ATM, etc. are examples of layer 2 encapsulation types. If on one machine you had HDLC set (default I believe) but on the other side you had Frame Relay set up you would have a line protocol down output.

Hope this helps.