通俗地說線纜檢測可以檢測交換機接口到終端之間的網線情況。不管是思科還是華爲、華三，在線纜檢測時都使用了時域反射time domain reflectometer (TDR)技術。
通過時域反射TDR (Time Domain Reflectometry)檢測電纜狀態的原理如下：
交換機發送一個脈衝信號，脈衝信號在電纜中傳輸，如果遇到電纜末端或其它故障點，部分能量會被反射回來，這種現象稱爲時域反射。線纜檢測技術就是測量脈衝在電纜中傳輸、達到故障點及返回的時間，把測量到的時間轉換成距離。
華爲和華三稱之爲虛電纜檢測VCT（Virtual Cable Test）技術。
一、下面先以思科爲例說一下：
測試環境一：思科3560-CG，一臺8口千兆POE交換機
1、交叉線測試：
SDYC-UCDEMO_SW1#test cable-diagnostics tdr interface g0/8
TDR test started on interface Gi0/8
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
SDYC-UCDEMO_SW1#show cable-diagnostics tdr interface g0/8
TDR test last run on: March 30 02:30:30
Interface Speed Local pair Pair length Remote pair Pair status
Gi0/8 1000M Pair A 0 +/- 10 meters Pair B Normal
Pair B 0 +/- 10 meters Pair A Normal
Pair C 0 +/- 10 meters Pair C Normal
Pair D 0 +/- 10 meters Pair D Normal
2、直通線測試：
SDYC-UCDEMO_SW1#test cable-diagnostics tdr interface g0/8
TDR test started on interface Gi0/8
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
SDYC-UCDEMO_SW1#show cable-diagnostics tdr interface g0/8
TDR test last run on: March 30 02:32:09
Interface Speed Local pair Pair length Remote pair Pair status
Gi0/8 1000M Pair A 0 +/- 10 meters Pair B Normal
Pair B 0 +/- 10 meters Pair A Normal
Pair C 0 +/- 10 meters Pair D Normal
Pair D 0 +/- 10 meters Pair C Normal
3、錯誤線序測試：
SDYC-UCDEMO_SW1#test cable-diagnostics tdr interface g0/8
Link state may be affected during TDR test
TDR test started on interface Gi0/8
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
SDYC-UCDEMO_SW1#show cable-diagnostics tdr interface g0/8
TDR test last run on: March 30 02:34:36
Interface Speed Local pair Pair length Remote pair Pair status
Gi0/8 100M Pair A N/A Pair B Normal
Pair B N/A Pair A Normal
Pair C N/A Pair D Normal
Pair D N/A Pair C Normal
這條線的線序雖然是錯的，但是是通的，所以後面顯示的狀態都是normal
4、線序超有問題的測試：
SDYC-UCDEMO_SW1#show cable-diagnostics tdr interface g0/8
TDR test last run on: March 30 03:06:38
Interface Speed Local pair Pair length Remote pair Pair status
Gi0/8 auto Pair A N/A N/A Normal
Pair B 0 +/- 10 meters N/A Short/Crosstalk
Pair C 0 +/- 10 meters N/A Open
Pair D N/A N/A Normal
這條線纜是線序超有問題的，接上去根本不通的，這時可以看到Pair Status上面顯示的內容。
測試環境：一臺2960XR交換機
1、直通線測試：
Switch#show cable-diagnostics tdr int g1/0/24
TDR test last run on: November 12 21:56:11
Interface Speed Local pair Pair length Remote pair Pair status
Gi1/0/24 1000M Pair A 22 +/- 10 meters Pair A Normal
Pair B 22 +/- 10 meters Pair B Normal
Pair C 22 +/- 10 meters Pair C Normal
Pair D 22 +/- 10 meters Pair D Normal
同一條網線，這個顯示結果與3560-CG上有些不同。不過Pair的狀態都是正常的。
2、交叉線測試：
Switch#test cable-dia tdr int g1/0/1
TDR test started on interface Gi1/0/1
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
Switch#show cable-diag tdr int g1/0/1
TDR test last run on: November 12 21:58:42
Interface Speed Local pair Pair length Remote pair Pair status
Gi1/0/1 1000M Pair A 21 +/- 10 meters Pair B Normal
Pair B 21 +/- 10 meters Pair A Normal
Pair C 21 +/- 10 meters Pair C Normal
Pair D 21 +/- 10 meters Pair D Normal
3、線序超有問題的線纜測試：
Switch#test cable-diagnostics tdr int g1/0/5
TDR test started on interface Gi1/0/5
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
Switch#show cable-diagnostics tdr int g1/0/5
TDR test last run on: November 12 22:03:52
Interface Speed Local pair Pair length Remote pair Pair status
Gi1/0/5 auto Pair A N/A N/A Normal
Pair B N/A N/A Normal
Pair C 1 +/- 10 meters N/A Open
Pair D 0 +/- 10 meters N/A Open
雖然這個也與3560-CG上顯示的不同，不過都可以看出來這條線纜是有問題的。
需要說明一點，當我們進行TDR測試時，會影響到這條鏈路數據的傳輸，也就是說會導致該條線纜連接的客戶端或是設備斷網。
另外，它不支持對SFP口進行測試。
二、華爲VCT測試
由於手頭沒有測試設備，下面的命令是查看文檔所得
測試方法在接口模式下輸入virtual-cable-test，如下所示：
相關輸出的說明如下：
三、H3C的VCT測試
測試方法同華爲一樣，在接口模式下輸入virtual-cable-test
顯示結果如下：
相關輸出解釋如下：
在這裏需要補充一下，這三家並不是所有的交換機都支持VCT，如果需要請查看相關文檔。
還有就是這三家的VCT都只是針對網線的測試，不要用來測試光纜！