A Time Domain Reflectometry (TDR) is an electrical instrument that uses time-domain reflectometry to characterize and locate faults in metallic cable (for example, twisted pair wire or coaxial cable). It can also be used to locate discontinuities in a connector, printer circuit board, or any other electrical path. The equivalent device for optical fiber is an optical time-domain reflectometry. These concepts are implemented in the Networking Switches and Routers instead of Custom built devices like Fluke Tester or Scanner like (DTX 1800 or DSP 4100). Though the Output from Switches is not as detailed or as insightful as those from such Scanners, but it is very efficient in-house diagnostic tool for Network Engineers. This Article will give a broad over view of the network diagnostic using TDR commands and the supported Network Switches.
A Word of Caution: in most of the switches, the ports will go down while testing, causing disruption to Network, so care should be taken in the production environment. Please verify the OS Versions which support TDR, before trying.
Some of the TDR supporting Switch Models (as on 12th August, 2016)
| Cisco Switches | HP Switches | Juniper Switches | |
|---|---|---|---|
| Cisco Catalyst 2960 | HP 5400R ZL2 switch | Juniper EX2200 Switch | |
| Cisco Catalyst 2960G | Cisco Catalyst 2960G | Juniper EX3200 Switch | |
| Cisco Catalyst 3806G | HP 5406R Switch | Juniper EX3300 Switch | |
| Cisco Catalyst 3560E&60X | Cisco Catalyst 3560E&60X | Juniper EX4200 Switch | |
| Cisco Nexus 7000 & 9000 | HP 5500 EI Switch |
Diagnostics using TDR
Network Engineer can locate the point of failure in the connectivity like faulty cable or pairs cut. Network diagnostic is a tool that is used in diagnosis and troubleshooting of network problem.
Switches or Straight cable being used can be checked using TDR Commands, which is useful in diagnosis of Network issues.
In Cisco Switches, if we test Giga Ethernet interface all pair should be ‘normal’, if the output is ‘open circuit’, still it is working (on a downgraded speed) which means that at least one pair is open or disconnected. Re-crimping on both ends may solve the problem and the speed increases to Gigabit.
If there is any Fail, short, impedance mismatch as errors, in Cisco TDR interface output, clearly an indication of bad cabling (either patch cable or horizontal cable). It is time to replace or repair the Cabling.
Some of the Switches like that of Juniper and HP, also provide Propagation delay between pairs. For example HP output calls it as Pair Skew and Juniper calls it as Skew time. Juniper output is of Skew Time is the difference between the least pair and the measured pair. Propagation delay is useful in diagnosing port related problems. The cause could be electronics or the Connectors or mating mismatch. Ideally it should be equal across the pairs or zero in some switches, like Juniper.
HP and Juniper also provide Polarity between pairs on either ends, though in different forms. The Juniper reports it as Polarity swap and HP as Pair polarity. Automatic polarity detection and correction permits automatic adjustment of wiring errors on all RJ45 ports. The cause could be Cable pair mismatches.
The possible problems due to pair cut could be downgraded speed (Gigabit to Fast Ethernet 100MBps), which is usually reported by users as Network being slow.
Features and limitations on Cisco Switches and Routers
- Switches need to run IOS version 12.2 or later. TDR is supported in IOS version 15.0. IOS version below 12.2 like 12.0 and 12.1 do NOT support TDR
- TDR is supported only on copper Ethernet 10/100/1000 ports. It is not supported on 10/100 ports or small form-factor pluggable (SFP) module ports.
- In Cisco, TDR can test cable lengths up to a maximum of 155 meters.
- TDR works on 10/100/1000Base Tx. Fiber optic ports and TenGigabitEthernet copper ports are not supported as on the date of this article.
Command:
Cisco SW# Show cable-diagnostics tdr interface gi0/48
Features and Limitations on HP Switches
- The Virtual Cable Test (VCT) uses the same commands as TDR. It is only for Giga transceivers like copper transceiver (J8177C – Procure Gigabit 1000Base-T Mini-GBIC).
- The VCT supported only on copper Giga Ethernet 1000Base ports. Internal Ethernet ports and Fiber ports are not supported.
- Virtual Cable Test (VCT) test result includes cable distance to the fault, but not the cable length.
- Virtual Cable Test (VCT) displays the cable condition like Normal or Abnormal status.
Commands:
[Sysname]# Interface gigabit Ethernet 1/0/1
Sysname-GigabitEthernet 1/0/1]# Virtual-c
Features and Limitations on Juniper Switches
Juniper Switches need to run Junos OS 12.3. TDR is supported in Junos 14.1X53, 14.2, 15.1 Versions. Lower OS Versions like Junos OS 12.2R4.5, 12.2R3.5, 12.2R2.5 do not support TDR.
- TDR Support EX2200, EX3200, EX3300, and EX4200 switches – RJ-45 network interfaces. The TDR test is not supported on management interface and SFP interfaces.
- EX6200 and EX8200 Switches – RJ-45 interface on line cards.
- It can detect a broken twisted pair and provide the approximate distance to the break. It can also detect polarity swaps, pair swaps, and excessive skew.
Command:
User@switch> request diagnostics tdr start interface ge-0/0/1