Investigating Cisco's built-in TDR
I love finding stuff like this. In the course of my daily Internet browsing, today I came across this post from a while back on the CCNP Recertification blog. It turns out a number of Cisco switches (apparently the 3560/3750, and some 4500 and 6500 modules) have a TDR embedded in each interface. How cool is that?
Cisco's documentation shows the simple but very handy two-part command syntax.
Switch# test cable-diagnostics tdr interface g0/1
TDR test started on interface Gi0/1
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
Switch# sh cable-diagnostics tdr int g0/1
TDR test last run on: March 01 00:09:06
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/1 auto Pair A 20 +/- 4 meters N/A Open
Pair B 20 +/- 4 meters N/A Open
Pair C 21 +/- 4 meters N/A Open
Pair D 20 +/- 4 meters N/A Open
This output shows a cable approximately 20 meters in length connected to interface G0/1, with nothing attached to the far end. If you're like me, your first thought is "very cool, but is it accurate?" I grabbed a couple spare cables of various lengths and set about an unscientific benchmark. I measured each cable in three manners: by hand, with my Fluke TDR, and with a Catalyst 3560G-24PS-S. My observed results are below.
Cable Length Fluke 3560 ---------------------------------------------- A 69 ft 69 ft 20 +/- 4m (~66 ft) B 21 ft 21 ft 6 +/- 4m (~20 ft) C 83 ft 83 ft 25 +/- 4m (~82 ft)
As you can see, the 3560 is impressively accurate, even without its allowed 4m variance. My next consideration was whether a TDR test would disrupt interface operation. Trying it on a live port while running a constant ping yielded no observable effect, not even a brief disconnection pop-up. This is clearly a very handy tool. I only wish Cisco would do more to publicize cool features like this.
About the Author
Jeremy Stretch is a freelance networking engineer, instructor, and the maintainer of PacketLife.net. He currently lives in Fairfax, Virginia, on the edge of the Washington, DC metro area. Although primarily an R&S guy, he likes to get into everything, and runs a free network training lab out of his basement for fun. You can contact him by email or follow him on Twitter.
Comments
On a 7600 with WS-X6748-GE-TX, there is disruption on the traffic :(. On a 3750 i didn't see any.
Definately saw a drop on my 3560, also got the following result:
CNPAS226#sh cable-diagnostics tdr int g0/1
TDR test last run on: November 14 18:06:36
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- -----------
Gi0/1 100M Pair A N/A Pair A Normal
Pair B N/A Pair B Normal
Pair C 180 +/- 2 meters Pair C Short
Pair D 164 +/- 2 meters Pair D Short
That's a really cool feature. Seems to work only with gig interfaces, though - bummer :)
This works on one of my 2960's with 100Mb too. Although I did have something plugged into it far side - no packets dropped on testing. Impressed!
Cisco IOS Software, C2960 Software (C2960-LANBASE-M), Version 12.2(25)SEE3, RELE ASE SOFTWARE (fc2)
DARWIN_SW2#sh cable-diagnostics tdr int fa0/3
TDR test last run on: June 29 00:44:25
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/3 100M Pair A 0 +/- 15 meters Pair A Normal
Pair B 0 +/- 15 meters Pair B Normal
Pair C N/A Pair C Not Supported
Pair D N/A Pair D Not Supported
On some of my 2960 switches, I get length "N/A" when I have lost connectivity to the end-node. Has anyone found what this means - other than the switch couldn't determine the length?
There was a drop on 6500 with 6748s as well. Should have scrolled down to the comments before trying it....
This is a list of some of the findings I have gotten from testing with a 2960 switch:
While testing a port with a POE attached:
POE will 'sometimes' add one meter to length, with or without power.
Normal port with nothing attached and a 3ft patch cord: (sometimes with poe attached)
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/2 auto Pair A 1 +/- 1 meters N/A Open
Pair B 1 +/- 1 meters N/A Open
Pair C N/A N/A Not Supported
Pair D N/A N/A Not Supported
Normal port with 5 ft patch cord: (also with 3ft patch cord and POE attached)
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/2 auto Pair A 2 +/- 1 meters N/A Open
Pair B 2 +/- 1 meters N/A Open
Pair C N/A N/A Not Supported
Pair D N/A N/A Not Supported
Normal with 5ft patch cord and POE attached:
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/2 auto Pair A 3 +/- 1 meters N/A Open
Pair B 3 +/- 1 meters N/A Open
Pair C N/A N/A Not Supported
Pair D N/A N/A Not Supported
Normal with 42ft cable and device plugged in:
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/2 100M Pair A 15 +/- 15 meters Pair A Normal
Pair B 15 +/- 15 meters Pair B Normal
Pair C N/A Pair C Not Supported
Pair D N/A Pair D Not Supported
I have not found a definite reason for the N/A response on active ports (Pair A, Pair B) but it is usually due to a defective NIC.
Hope this helps!
I've done some investigation with regard to using a Gigabit Crossover cable and a Straight Through cable and TDR on a Cisco 2960, and came across some interesting things.
PART I:
Tests using a Crossover cable, type "Gigabit T568B crossover, All four pairs crossed" (see Wikipedia) of 2 meter in length:
1) From Gig0/1 on 2960 to a Gig0/23 port on 3750:
2960#show cable-diagnostics tdr interface gigabitEthernet 0/1
TDR test last run on: April 16 03:44:36
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/1 1000M Pair A 0 +/- 10 meters Pair A Normal
Pair B 0 +/- 10 meters Pair B Normal
Pair C 0 +/- 10 meters Pair C Normal
Pair D 0 +/- 10 meters Pair D Normal
2960#
2) From Gig0/1 on 2960 to Gig8 port on Linksys SRW2016:
2960#show cable-diagnostics tdr interface gigabitEthernet 0/1
TDR test last run on: April 16 03:47:05
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/1 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
2960#
Notice the difference in remote pairs!
3) From Gig0/1 on 2960 to 10/100 port on a CPE device:
2960#show cable-diagnostics tdr interface gigabitEthernet 0/1
TDR test last run on: April 16 03:45:09
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/1 auto Pair A N/A Pair A Normal
Pair B N/A Pair B Normal
Pair C 2 +/- 2 meters Pair C Short
Pair D 1 +/- 2 meters Pair D Short
2960#
4) From Fa0/40 on 2960 to 10/100 port on a CPE device:
2960#show cable-diagnostics tdr interface fastEthernet 0/40
TDR test last run on: April 16 04:16:47
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/40 100M Pair A 0 +/- 15 meters Pair A Normal
Pair B 0 +/- 15 meters Pair B Normal
Pair C N/A Pair C Not Supported
Pair D N/A Pair D Not Supported
2960#
PART II:
Then, all tests were repeated with a Straight-Through cable of 3 meter in length.
1) From Gig0/1 on 2960 to a Gig0/23 port on 3750:
2960#show cable-diagnostics tdr interface gigabitEthernet 0/1
TDR test last run on: April 16 03:50:16
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/1 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
2960#
2) From Gig0/1 on 2960 to Gig8 port on Linksys SRW2016:
2960#show cable-diagnostics tdr interface gigabitEthernet 0/1
TDR test last run on: April 16 03:48:46
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/1 1000M Pair A 0 +/- 10 meters Pair A Normal
Pair B 0 +/- 10 meters Pair B Normal
Pair C 0 +/- 10 meters Pair C Normal
Pair D 0 +/- 10 meters Pair D Normal
2960#
3) From Gig0/1 on 2960 to 10/100 port on a CPE device:
2960#show cable-diagnostics tdr interface gigabitEthernet 0/1
TDR test last run on: April 16 03:51:37
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/1 100M Pair A N/A Pair A Normal
Pair B N/A Pair B Normal
Pair C 2 +/- 2 meters Pair C Short
Pair D 2 +/- 2 meters Pair D Short
2960#
4) From Fa0/40 on 2960 to 10/100 port on a CPE device:
2960#show cable-diagnostics tdr interface fastEthernet 0/40
TDR test last run on: April 16 04:18:15
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/40 100M Pair A 4 +/- 15 meters Pair B Normal
Pair B 4 +/- 15 meters Pair A Normal
Pair C N/A Pair D Not Supported
Pair D N/A Pair C Not Supported
2960#
Can this get the full length of a cable as it is connect from the switch ---> to the patch panel ---> to the user office ?