E1 Link problem

Unanswered Question
Sep 10th, 2008

I have an 2Meg E1 link terminating on both on RAD HTU E1 modems. The link is frequently flapping up & down but the interface on the router does not go down (only the line protocol) as shown below:

R#show int Serial2/0:0

Serial2/0:0 is up, line protocol is down

Hardware is DSX1

Description: Kenstream link to Kenyatta Avenue

Internet address is

MTU 1500 bytes, BW 1984 Kbit, DLY 20000 usec,

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

Encapsulation PPP, LCP Listen

Closed: IPCP, loopback not set

Last input 00:56:33, output 00:04:38, output hang never

Last clearing of "show interface" counters 00:01:30

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

Queueing strategy: weighted fair

Output queue: 0/1000/64/0 (size/max total/threshold/drops)

Conversations 0/258/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

Available Bandwidth 1488 kilobits/sec

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

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

0 packets input, 0 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

4 packets output, 56 bytes, 0 underruns

0 output errors, 0 collisions, 5 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

Timeslot(s) Used:1-31, Transmitter delay is 0 flags

5R#show runn int s2/0:0

Building configuration...

Current configuration : 136 bytes


interface Serial2/0:0


ip address

encapsulation ppp

Any advice in this regard?

I have this problem too.
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Giuseppe Larosa Wed, 09/10/2008 - 01:56

Hello Sammy,

I see

Encapsulation PPP, LCP Listen

Closed: IPCP, loopback not set

Last input 00:56:33, output 00:04:38, output hang never

the router interface is not receveing any input in the last 56 minutes.

reset the HTU modem and see if this is enough.

If not contact the other end of the link to see if their router is powered on and working.

Have them to reset their DCE3 HTU.

If even this doesn't solve open a ticket with the wan provider and have them to perform the loopback tests

Hope to help


njorosam Wed, 09/10/2008 - 02:18

Thanks Giuseppe,

Though the link is maintained by the local telco, it's set such that our modems connect back-to-back with each other without going through the SP equipment. The distance is about 200 meter in City center. Whenever i reset the DTUs in either side, the link will come up and stay up for about 1hr, the go down again. The interface reset counters are also incementing (what does this mean).

Giuseppe Larosa Wed, 09/10/2008 - 08:46

Hello Sammy,

when connecting two modems in back to back without any telco equipment in the middle one of the two should provide the clock and the other one should accept the clock on the line.

Otherwise if every modem uses its own clock after some time they become so different that they sample the line at the wrong time causing issues.

Because these are synchronus serial interfaces the clock has to be in sync on both ends.

I would check how the two modems are configured if their configuration is identical one of them has to be changed to accept clock on the line.

Hope to help



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