Setting Up a BERT Test on a T1 Interface

Problem

You want to run a bit error rate test (BERT) to test the quality of a T1 link.

Solution

First, configure the BERT test parameters on the local interface:

	[edit interfaces t1-0/0/3 ]
	aviva@RouterF# set t1-options 
bert-algorithm repeating-3-in-24 
	aviva@RouterF# set t1-options bert-period 180 
	aviva@RouterF# set disable 

Then, put the interface at the far end of the link into loopback:

	[edit interfaces t1-0/0/3 ]
	aviva@RouterE# set t1-options loopback remote 

Then run the BERT test on the local router:

	aviva@RouterF> test interface t1-0/0/3 t1-bert-start

Use the following command to stop a BERT test:

	aviva@RouterF> test interface t1-0/0/3 t1-bert-stop

 

Discussion

BERT testing checks the quality of a link by sending out a known bit pattern and verifying that the received pattern matches. Bit errors occur when the pattern doesn't match and indicates noise or other problems on the physical circuit. The higher the bit error rate, the worse the problem. The link you are testing is in loopback mode so the transmitted packets are looped back to the same interface.

This recipe shows how to configure BERT on a local T1 interface. You can also configure it similarly on other interfaces, including T3, E1, E3, DS, OC3, OC12, and STM1. Choose a bit pattern to send out in the set bert-algorithm command. There are about 20 patterns, including all ones, all zeros, alternating ones and zeros, and setting one bit out of a group of bits. Use the set t1-options bert-algorithm ? command to see the full list. In this recipe, we set 3 bits out of each 24 bits. By default, a BERT test runs for 10 seconds. We change this to 3 minutes (180 seconds). Finally, the T1 interface must be disabled for the BERT test to work.

On the remote end, the link must be in remote loopback mode. Verify that the remote end of the link is in remote loopback mode:

	aviva@RouterE> show interfaces t1-0/0/3
	Physical interface: t1-0/0/3, Enabled, Physical link is Up
	 Interface index: 140, SNMP ifIndex: 38
	 Link-level type: PPP, MTU: 1504, Clocking: Internal, Speed: T1,
	 Loopback: Remote, FCS: 16, Framing: ESF
	 Device flags : Present Running
	 Interface flags: Point-To-Point SNMP-Traps 16384
	 Link flags : Keepalives
	 Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3
	 Keepalive: Input: 0 (never), Output: 0 (never)
	 LCP state: Conf-req-sent
	 NCP state: inet: Down, inet6: Not-configured, iso: Not-configured, mpls:
	 Not-configured
	 CHAP state: Not-configured
	 CoS queues : 8 supported
	 Last flapped : 2005-05-26 10:21:08 PDT (00:00:07 ago)
	 Input rate : 0 bps (0 pps)
	 Output rate : 72 bps (0 pps)
	 DS1 alarms : None
	 DS1 defects : None

	Logical interface t1-0/0/3.0 (Index 69) (SNMP ifIndex 39)
	 Flags: Hardware-Down Point-To-Point SNMP-Traps Encapsulation: PPP
	 Protocol inet, MTU: 1500
	 Flags: Protocol-Down
	 Addresses, Flags: Dest-route-down Is-Preferred Is-Primary
	 Destination: 10.0.13/24, Local: 10.0.13.1, Broadcast: 10.0.13.255

The physical interface is marked Loopback: Remote. In addition, a number of fields in the logical interface section indicate that the T1 interface at the remote side of the link is down.

Run the BERT test from operational mode. Because this is a T1 interface, use the t1- bert-start option. For other interfaces, use the test interface ? command to determine the proper option.

The show interfaces extensive command shows the results of the BERT test. Here, the test is in progress:

	aviva@RouterF> show interfaces extensive t1-0/0/3 | find bert
	DS1 BERT configuration:
	 BERT time period: 180 seconds, Elapsed: 96 seconds (in progress)
	 Induced Error rate: 10e-0, Algorithm: 3 in 24, Repetitive (26)
	 
Bit count : 146557240
	 Error bit count: 0
	…

The following output shows the results of the completed test, which found no bit errors on the T1 link:

	aviva@RouterF> show interfaces extensive t1-0/0/3 | find bert
	DS1 BERT configuration:
	 BERT time period: 180 seconds, Elapsed: 180 seconds (completed)
	 Induced Error rate: 10e-0, Algorithm: 3 in 24, Repetitive (26)
	 Bit count : 273186232
	 Error bit count: 0
	…

You can configure an error rate to force errors in the bit stream:

	[edit interfaces t1-0/0/3]
	aviva@RouterF# set t1-options 
bert-error-rate 3

This configuration generates one error in every 1,000 packets (10e-3). The show interfaces command output shows the number of errors and the error rate:

	aviva@RouterF> show interfaces t1-0/0/3 extensive | find 
bert
	 DS1 
BERT configuration:
	 BERT time period: 10 seconds, Elapsed: 10 seconds (completed)
	 Induced Error rate: 10e-3, Algorithm: 3 in 24, Repetitive (26)
	 Bit count : 15369016
	 Error bit count: 15355
	 Error rate: 10e-3.0