Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

LoopBack Testing

I would like to know how to test a loopback when the telco has given me the loop? Is there any configuration needed on the router to see the loopback coming from the telco? thanks

This is the serial interface

ROUTER#sh int s0/1/0

Serial0/1/0 is up, line protocol is down

  Hardware is WIC MBRD Serial

  Description: *** LINK TO INNOVE: IC-AEZ-7992 ***

  Internet address is

  MTU 1500 bytes, BW 256 Kbit/sec, DLY 20000 usec,

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

  Encapsulation FRAME-RELAY IETF, loopback not set

  Keepalive set (10 sec)

  LMI enq sent  64845, LMI stat recvd 62572, LMI upd recvd 0, DTE LMI down

  LMI enq recvd 0, LMI stat sent  0, LMI upd sent  0

  LMI DLCI 0  LMI type is ANSI Annex D  frame relay DTE  segmentation inactive

  FR SVC disabled, LAPF state down

  Broadcast queue 0/64, broadcasts sent/dropped 12135/0, interface broadcasts 11


  Last input 06:19:18, output 00:00:08, output hang never

  Last clearing of "show interface" counters 1w0d

  Input queue: 0/75/0/98 (size/max/drops/flushes); Total output drops: 4039111

  Queueing strategy: weighted fair

  Output queue: 0/1000/64/4039111 (size/max total/threshold/drops)

     Conversations  0/21/256 (active/max active/max total)

     Reserved Conversations 0/0 (allocated/max allocated)

     Available Bandwidth 192 kilobits/sec

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

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

     2067246 packets input, 1100838086 bytes, 0 no buffer

     Received 0 broadcasts (0 IP multicasts)

     1417 runts, 0 giants, 0 throttles

     4531 input errors, 323 CRC, 0 frame, 0 overrun, 0 ignored, 2791 abort

     1878089 packets output, 236373863 bytes, 0 underruns

     0 output errors, 0 collisions, 751 interface resets

     0 unknown protocol drops

     0 output buffer failures, 0 output buffers swapped out

     8 carrier transitions

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

Hall of Fame Super Gold

LoopBack Testing

Serial0/1/0 is up, line protocol is down

Your telco didn't have the loop set.

If there was a loop, it would be Layer 1 is DOWN but protocol would be UP. 

New Member

LoopBack Testing

hi leo, so which means i don't see the loop. what will i do to see the loop? also is there configuration needed?

Hall of Fame Super Gold

LoopBack Testing

Loopback Tests for T1/56K Lines

Don't forget to rate our useful posts. 

New Member

LoopBack Testing

the serial interface is FRAME-RELAY IETF encapsulation. the link is about hdlc.

Hall of Fame Super Gold

LoopBack Testing

It doesn't matter.  As long as you can run the "diagnostics loopback" command on the interface level.

Make sure you do not do this command on the remote end.  Always do this on the local-end router.

New Member

LoopBack Testing

is this the command that could see loopback coming from telco?

Router#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#controller t1 0
Router(config-controller)#no pri-group timeslots 1-24
Router(config-controller)#channel-group 0 timeslots 1-24 speed 64

!--- This automatically creates a single Serial0:0 interface. 

Router(config-controller)#loopback local

!--- The loopback local command above is only necessary for software loopbacks.

Router(config)#interface serial 0:0 
Router(config-if)#encapsulation hdlc
Hall of Fame Super Gold

Re: LoopBack Testing

Should be.  Never tried the loopback commands on a router.

Have done loopback tests on Nortel Passports/DPN 100.  Even did them with PLDT.

New Member

LoopBack Testing

i've never tried seeing loopback from the router also. i think it shows only in the serial interface status

LoopBack Testing


better ask telco for a loop instead of the reverse. you should see up/up (looped) on the show interface serial command output.

ask telco from which node they're giving the loop from, then ping the same IP on the router, and check for errors or drops.