Performing a Loopback Test on a T1 Interface

Problem

A new T1 interface has been installed and configured, and you want to perform a loopback test to make sure the interface is operating properly.

Solution

Set up a T1 loopback configuration:

	[edit interfaces t1-0/0/3 ]
	aviva@RouterF# set t1-options loopback local 
	aviva@RouterF# set no-keepalives 
	aviva@RouterF# set encapsulation cisco-hdlc 
	aviva@RouterF# commit 

Make sure the interface is up:

	aviva@RouterF> show interfaces t1-0/0/3 terse
	Interface Admin Link Proto Local Remote
	t1-0/0/3 up up

Then zero all the statistics on the interface:

	aviva@RouterF> clear interfaces statistics t1-0/0/3

Run the loopback test, which sends 1,000 ping messages rapidly and reports the results in a single message:

	aviva@RouterF> ping interface t1-0/0/3 10.0.13.2 
bypass-routing count 1000 rapid
	PING 10.0.13.2 (10.0.13.2): 56 data bytes
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	--- 10.0.13.2 ping statistics --
	1000 packets transmitted, 1000 packets received, 0% packet loss
	round-trip min/avg/max/stddev = 2.376/19.102/369.920/35.795 ms

 

Discussion

To perform a loopback test on the T1 interface, first configure the T1 interface to use local loopback mode. Also include the encapsulation cisco-hdlc and no-keepalives statements to force the interface to stay up so you can run the ping loopback test. Then you need to create a physical loopback at the T1 port by connecting a T1 loopback plug to the T1 port. You can make a T1 loopback plug by connecting pin 1 to pin 4 and pin 2 to pin 5 on an RJ-48 plug.

The first commands in this recipe configure the loopback. After making sure the interface is up and zeroing all interfaces statistics, run the loopback test, which sends 1000 ping messages rapidly and reports the results in a single message. Include the bypass-routing option in the ping command to directly ping a system on an attached network, bypassing normal routing tables. This option forces the packets to be transmitted out the interface, because they have a local destination address and from an IP point of view, they are already at their destination.

If there are any problems on the link, you see input and output errors in the show interfaces extensive command output. This output also shows that 1,000 ping messages were sent (in Output packets) and 1,000 responses were received (in Input packets).

	aviva@RouterF> show interfaces t1-0/0/3 extensive
	Physical interface: t1-0/0/3, Enabled, Physical link is Up
	 Interface index: 140, SNMP ifIndex: 38, Generation: 21
	 Description: J2300 T1 line in local office
	 Link-level type: Cisco-HDLC, MTU: 1504, Clocking: Internal, Speed: T1,
	 
Loopback: Local, FCS: 16, Framing: ESF
	 Device flags : Present Running
	 Interface flags: Point-To-Point SNMP-Traps 16384
	 Link flags : No-Keepalives
	 Hold-times : Up 0 ms, Down 0 ms
	 CoS queues : 8 supported
	 Last flapped : 2005-05-25 20:57:42 PDT (00:02:13 ago)
	 Statistics last cleared: 2005-05-25 20:58:04 PDT (00:01:51 ago)
	 Traffic statistics:
	 Input bytes : 84000 0 bps
	 Output bytes : 89000 0 bps
	 Input packets : 1000 0 pps
	 Output packets: 1000 0 pps
	 Input errors:
	 Errors: 0, Drops: 0, Framing errors: 0, Policed discards: 0,
	 L3 incompletes: 0, L2 channel errors: 0, L2 mismatch timeouts: 0,
	 HS link CRC errors: 0, SRAM errors: 0, Resource errors: 0
	 Output errors:
	 Carrier transitions: 0, Errors: 0, Drops: 0, Aged packets: 0, MTU errors: 0,
	 Resource errors: 0
	 Queue counters: Queued packets Transmitted packets Dropped packets
	 0 best-effort 1000 1000 0
	 1 expedited-fo 0 0 0
	 2 assured-forw 0 0 0
	 3 network-cont 1284 1284 0
	 DS1 alarms : None
	 DS1 defects : None
	 T1 media: Seconds Count State
	 SEF 0 0 OK
	 BEE 0 0 OK
	 AIS 0 0 OK
	 LOF 0 2 OK
	 LOS 0 1 OK
	 YELLOW 2 1 OK
	 BPV 0 0
	 EXZ 1 1
	 LCV 1 1
	 PCV 0 0
	 CS 0 0
	 LES 1
	 ES 0
	 SES 0
	 SEFS 0
	 BES 0
	 UAS 0
	…

The later part of the command output shows any active alarms and T1 media defects, which you can also use to troubleshoot interface problems.