Line protocol looped?

Unanswered Question
Oct 15th, 2009

Hey guys

In my CCNP study lab I have a couple 2502's that I'm trying to connect with a back-to-back serial cable. Got a clockrate configured on the DCE end, they both have ppp encapsulation configured, except the line protocol isn't coming up.

It says:

R8#sh int s0

Serial0 is up, line protocol is down (looped)

Hardware is HD64570

Internet address is

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation PPP, LCP Listen, loopback not set

Keepalive set (10 sec)

Last input 00:00:04, output 00:00:04, output hang never

Last clearing of "show interface" counters 06:41:27

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

40 packets input, 560 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

40 packets output, 560 bytes, 0 underruns

0 output errors, 0 collisions, 4 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=up DSR=up DTR=up RTS=up CTS=up

What does that mean, line protocol is down (looped)? What causes that? How do I fix it?!?


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scottmac Thu, 10/15/2009 - 17:27

The back-to-back cable has a DTE end and a DCE end ... check to make sure you have them connected to the right ports (i.e., check the labels, and / or swap ends on the cable).

CriscoSystems Thu, 10/15/2009 - 17:29

Done. Configured a clockrate on the interface with the DCE end plugged into it.

Tried different cables; problem persists.

Giuseppe Larosa Fri, 10/16/2009 - 01:18

Hello Stuey,

looped means the router receives its own keepalives back.

you can verify using:

debug serial interface

debug PPP negotiation

the second debug should complain about a duplicated magic number detected.

you see that the number of input packets = number of output packets.

check the other router configuration and cable plugging.

see if any loop command is present on the other router interface.

Hope to help



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