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New Member

Serial interface problem..

Hi All,

I have a problem in a leased line, but while troubleshooting with the teleco if found out that when the make a HW loop i notice that my serial interface showing the following:

Serial13/1/7:0 is up, line protocol is down (looped) while using PPP encapsulation, so what is the problem?!!!

Below is the output for sh interface serial:


Serial13/1/7:0 is up, line protocol is down (looped)

Hardware is Multichannel E1

MTU 300 bytes, BW 128 Kbit, DLY 20000 usec,

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

Encapsulation PPP, crc 16, Data non-inverted

Keepalive set (10 sec)


Closed: CDPCP

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

Last clearing of "show interface" counters 00:00:14

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

14 packets input, 224 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

14 packets output, 224 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions no alarm present

Timeslot(s) Used:UNFRAMED, transmit delay is 0 flags



New Member

Re: Serial interface problem..


I belive this is the case with ppp encapsulation . In HDLC it will show even line protocol up when looped. Also for 14 seconds the loop statistics aregood. Try for more time and see what stats are there.


Aasheesh GUpta

New Member

Re: Serial interface problem..

Dear Aasheesh,

loop With HDLC it is Up Up (looped), my problem is that i can not get this link to be Up Up, it is always Up Down even without the loop, i am forced to use PPP, are there any suggestions?!!



Re: Serial interface problem..

The looped status is telling you that the interface sees a loopback facing it. The loop can be located anywhere in the path of the line. The first thing I would do is check your first point in house and verify the loop is not coming from you. Once you prove yourself good, Then verify that the loop is not coming from the remote site office.If you still have not found the loop then you can scream at the carrier to check their line.

New Member

Re: Serial interface problem..

Absolutely. PPP uses a pseudo-random magic number to identify its own LCP setup requests. If it sees setup requests coming from itself it will report a loop. My experience is that if PPP says there's a loop, you can trust it.

New Member

Re: Serial interface problem..

Yes there is a loop, but my question was why i see the interface status as Up Down even in the presence of the loop?!!!