工作点滴4 - 思科设备环路测试 - Cisco Loopback Tests

工作上常常会遇到运营商提供的专线断网的情况，在排障的时候经常会用环路测试来断定故障的位置。

 环路测试（loop test）：通过软件或硬件的方法把指定线路的信号、数据发回到发送方。通过在一条线路不同跳做环路来确定故障位置。

 

 

举个例子：

A 到 B 经过两个运营商的网络的专线进行互连。运营商B 的网络出了问题。我们做为在site B的工程师怎么和运营商的工程师一起判断哪里出了问题？

1. 初步检查自己的设备，跳线无破损松动等等

2. 通知运营商A线路有问题，要求环路测试，在远端NIU上面向site B做软环

3. 在site B的 wan端口能看到端口状态为（looped）

4. 要求运营商A把loop1去掉，在他们和运营商B的交接处（demarc point）放置软环，面向siteB

5. 依然在site B WAN 端口看到 （looped），这时可要求运营商B放置环路在离site B最近的交换站 （LME=last mile exchange；美国很时候称之为 CO = central office）

6. 这时在site B 的 WAN端口看不到环路，则说明问题出在运营商B

7. 要求运营商B进一步内部排错

Cisco Loopback Tests

• Loopback Modes
  
• BERT Tests

Loopback Modes

There are three main loopback modes as follows: local, network (both line and payload), and remote (line and payload). Specify theloopbackformat using theloopback [local |network |remote] command.

Examples of specific loopback modes follow:

• Set the first T1 into local loopback as follows:

  Router# config tEnter configuration commands, one per line. End with CNTL/Z.Router(config)# interface serial 3/0:1Router(config-if)# loopback local

  In the preceding example, local loopback loops the router output data back toward the router at the T1 framer and sends an AIS signal out toward the network.
  
  

• Set the first T1 into network line loopback as follows:

  Router# config tEnter configuration commands, one per line. End with CNTL/Z.Router(config)# interface serial 3/0:1Router(config-if)# loopback network line

  In the preceding example, network line loopback loops the data back toward the network (before the T1 framer).
  
  

• Set the first T1 into network payload loopback as follows:

  Router# config tEnter configuration commands, one per line. End with CNTL/Z.Router(config)# interface serial 3/0:1Router(config-if)# loopback network payload

  In the preceding example, network payload loopback loops just the payload data back toward the network at the T1 framer.
  
  

• The syntax of the loopback remote command follows:

  loopback [remote {line {fdl ansi | inband} | payload fdl ansi}]

  Set the first T1 into remote line inband loopback, as follows:
  

  Router# config tEnter configuration commands, one per line.  End with CNTL/Z.Router(config)# int serial 3/0:1Router(config-if)# loop remote line inband

  In the preceding example, remote line inband loopback sends a repeating 5-bit inband pattern (of 00001) to the remote end requesting that it enter into a network line loopback.
  
  Set the first T1 into remote line FDL ANSI bit loopback, as follows:
  

  Router# config tEnter configuration commands, one per line.  End with CNTL/Z.Router(config)# int serial 3/0:1Router(config-if)# loop remote line fdl ansi

  In the preceding example, remote line FDL ANSI bit loopback sends a repeating 16-bit ESF data link code word (of 00001110 11111111) to the remote end requesting that it enter into a network line loopback.
  
  Set the first T1 into remote payload FDL ANSI bit loopback, as follows:
  

  Router# config tEnter configuration commands, one per line.  End with CNTL/Z.Router(config)# int serial 3/0:1Router(config-if)# loop remote payload fdl ansi

  In the preceding example, remote payload ANSI bit loopback sends a repeating 16-bit ESF data link code word (of 00010100 11111111) to the remote end requesting that it enter into a network payload loopback

The Measurements in output to troubleshoot a serial link are :

• Reliability: Reliability of the interface as a fraction of 255 where 255/255 is 100% is calculated as an exponential average over five minutes.
• Input Errors : Sum of all errors that prevented the receipt of datagrams.
• CRC Error : Stands for Cyclic redundancy Check-sum, CRC counts increases whenever CRC mismatch is there or when a far end abort occurs and when the idle flag is corrupted.
• Frame error : When a packet doesn’t end on a 8 bit boundary.
• Abort : Number of packet whose receipt was aborted.

Checking Bit Errors Using a BERT

To check bit errors using a bit error test (BERT), use the following command:

[no] t1 t1 channel bert pattern {2^15 | 2^20 | 2^23 | 0s | 1s} interval minutes

where t1 channel is 1-28, 2^15 is an exponential number that represents a pseudo-random repeating pattern that is 32767 bits long,2^20 is a pseudo-random repeating pattern that is 1048575 bits long,2^23 is a pseudo-random repeating pattern that is 8388607 bits long, 0s is a pattern of all zeroes (00000000...),1s is a pattern of all 1s (111111...), andminutesare1-14400, which designate the time the BERT will run. To stop the BERT, use the optionalno form of the command. Following is an example of this command, where the pseudo-random pattern 2^20 is sent and repeats on the first T1 channel for 60 minutes:

Router# t1 1 bert pattern 2^20 interval 60

Note that the BERT command is not saved in NVRAM. The test patterns from the PA-CT3/4T1 port adapter are framed test patterns; therefore, they are inserted into the payload of a framed T1 signal.

To display the BERT results, use the following EXEC commands: sh cont t3number orsh cont t3numberbrief. Following are examples of displaying BERT results output:

1. Display BERT output results during a test, as follows:

   Router# sh cont t3 5/0 bT3 5/0 is up.CT3 H/W Version : 4, CT3 ROM Version : 0.116, CT3 F/W Version : 20.2.0  Mx H/W version : 2, Mx ucode ver : 1.25  Applique type is Channelized T3  No alarms detected.  FEAC code received : No code is being received  Framing is M23, Line Code is B3ZS, Clock Source is Internal.  T1 1 is down, speed: 1536 kbs, non-inverted data  timeslots: 1-24  FDL per ANSI T1.403 and AT&T 54016 spec.  Configured for FDL Remotely Line Looped  No alarms detected.  Framing is ESF, Line Code is B8ZS, Clock Source is Internal.  BERT test result (running)      Test Pattern : All 0's, Status : Sync, Sync Detected : 1      Interval : 4 minute(s), Time Remain : 4 minute(s)      Bit Errors(Sync BERT Started) : 0 bits      Bit Errors(Sync last Sync) : 0 bits , Bits Received : 7 Mbits

   Note When the T1 is under BERT, its line state is down. When under BERT (in the running state), the "Total Bit Errors" value is not valid if the "Status" field is "Not Sync."

2. When the test is done, display BERT results output as follows:

   Router# sh cont t3 5/0 bT3 5/0 is up.  CT3 H/W Version : 4, CT3 ROM Version : 0.116, CT3 F/W Version : 20.2.0  Mx H/W version : 2, Mx ucode ver : 1.25  Applique type is Channelized T3  No alarms detected.  FEAC code received : No code is being received  Framing is M23, Line Code is B3ZS, Clock Source is Internal.  T1 1 is up, speed: 1536 kbs, non-inverted data  timeslots: 1-24  FDL per ANSI T1.403 and AT&T 54016 spec.  Configured for FDL Remotely Line Looped  No alarms detected.  Framing is ESF, Line Code is B8ZS, Clock Source is Internal.  BERT test result (done)      Test Pattern : All 0's, Status : Not Sync, Sync Detected : 1      Interval : 4 minute(s), Time Remain : 0 minute(s)      Bit Errors(Sync BERT Started) : 0 bits      Bit Errors(Sync last Sync) : 0 bits , Bits Received : 368 Mbits

   Note The "Status" field is irrelevant when the BERT is done. If the "Sync Detected" counter is 0, the Bit Errors fields have no meaning.

3. When the test is stopped prematurely, display BERT results output as follows:

   Router# sh cont t3 5/0 bT3 5/0 is up.  CT3 H/W Version : 4, CT3 ROM Version : 0.116, CT3 F/W Version : 20.2.0  Mx H/W version : 2, Mx ucode ver : 1.25  Applique type is Channelized T3  No alarms detected.  FEAC code received : No code is being received  Framing is M23, Line Code is B3ZS, Clock Source is Internal.  T1 1 is up, speed: 1536 kbs, non-inverted data  timeslots: 1-24  FDL per ANSI T1.403 and AT&T 54016 spec.  Configured for FDL Remotely Line Looped  No alarms detected.  Framing is ESF, Line Code is B8ZS, Clock Source is Internal.  BERT test result (done)      Test Pattern : All 0's, Status : Not Sync, Sync Detected : 1      Interval : 4 minute(s), Time Remain : 2 minute(s) (unable to complete)      Bit Errors(Sync BERT Started) : 0 bits      Bit Errors(Sync last Sync) : 0 bits , Bits Received : 368 Mbits

   Note In the preceding three examples, the "Sync Detected" counter indicates the number of times the pattern sync is detected (from No Sync to Sync), the "Bit Errors(Sync BERT Started)" counter indicates the number of bit errors during BERT, and the "Bit Errors(Sync last Sync)" counter shows the number of bit errors since the last pattern sync is detected.

参考资料：

http://networking.ringofsaturn.com/Cisco/loopbacktests.php

http://technicafe.net/2012/05/troubleshooting-serial-link-through.html

 

29/4/2014 补充：

Metro Ethernet Loop Test

如果专线两端都是思科的设备，通过Metro Ethernet专线相连 （常见的Fast Ethernet和Gigbit Ethernet通过运营商的光纤网络以Lay2的形式就连接两端）

这时，对于设备来讲看到的就是一根网线或者光纤通过交换机把他们连起来。

例如，A - 运营商交换机- 运营商核心网 - 运营商交换机 - B （A和B用bgp相连）

 当运营商核心网出问题时，A和B看到的就是bgp邻居断了，但是他们与直接相连的交换机/猫 接口都是 up /up 这点和 frame relay via E1/T1 line是不一样的。

测试的时候可以让运营商在某点打环，面向A或B，然后在设备上 enable cdp

如果环路ok，那么你就能看到你cdp neighbor是你自己。

如果线路有问题，你看不到任何neighbor。