Cisco diagnostics

Stardate: 90666.54 Cisco switches and routers (Catalyst 3750 series in this example) support some really cool diagnostics. These diagnostics come in handy when trying to determine if a Layer 1 fault may be involved and where it is. This technology, known as Time domain reflectometry, is available on all Cisco 3750 models including the new Catalyst 3750X.
An initial example: This shows normal use an a no-fault return.

fab20a#test cable-diagnostics tdr interface Gi1/0/25
TDR test started on interface Gi1/0/25
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
fab20a#show cable-diagnostics tdr int Gi1/0/25
TDR test last run on: January 22 20:41:52

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/25  1000M Pair A     49   +/- 4  meters Pair A      Normal
                Pair B     45   +/- 4  meters Pair B      Normal
                Pair C     48   +/- 4  meters Pair C      Normal
                Pair D     45   +/- 4  meters Pair D      Normal

Another example: this shows normal use and an open circuit(most likely meaning no host is on the other side.

fab60a#test cable-diagnostics tdr interface GigabitEthernet 1/0/31
TDR test started on interface Gi1/0/31
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.

fab60a#show cable-diagnostics tdr int GigabitEthernet 1/0/31
TDR test last run on: January 08 14:45:40

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/31  auto  Pair A     3    +/- 4  meters N/A         Open
                Pair B     2    +/- 4  meters N/A         Open
                Pair C     0    +/- 4  meters N/A         Open
                Pair D     3    +/- 4  meters N/A         Open

Note that you must specify 'int' between tdr and the interface identifier. Presumably so you could shoot electrons at something that isn't an interface, like the door or something. An example of a broken pair:

fab20a#test cable-diagnostics tdr interface Gi1/0/25
TDR test started on interface Gi1/0/25
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
fab20a#show cable-diagnostics tdr int Gi1/0/25
TDR test last run on: January 22 18:33:07

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/25   100M Pair A     49   +/- 4  meters Pair A      Normal
                Pair B     45   +/- 4  meters Pair B      Normal
                Pair C     48   +/- 4  meters Pair C      Normal
                Pair D     0    +/- 4  meters Pair D      Open

Here the "D" pair is broken. You can see from the 'pair length' column that it is broken at the beginning of the cable. This means we got lucky. We were able to replace the patch cable instead of having a contractor rewire the wire in the conduit. Notice that with only 3 pairs, the link is still up at 100Mbit.