Administrator Guide

Maintenance Using TDR
The time domain reflectometer (TDR) is supported on all Dell EMC Networking switches.
TDR is an assistance tool to resolve link issues that helps detect obvious open or short conditions within any of the four copper
pairs. TDR sends a signal onto the physical cable and examines the reflection of the signal that returns. By examining the
reflection, TDR is able to indicate whether there is a cable fault (when the cable is broken, becomes unterminated, or if a
transceiver is unplugged).
TDR is useful for troubleshooting an interface that is not establishing a link; that is, when the link is flapping or not coming up.
TDR is not intended to be used on an interface that is passing traffic. When a TDR test is run on a physical cable, it is important
to shut down the port on the far end of the cable. Otherwise, it may lead to incorrect test results.
NOTE: TDR is an intrusive test. Do not run TDR on a link that is up and passing traffic.
To test and display TDR results, use the following commands.
1. To test for cable faults on the TenGigabitEthernet cable.
EXEC Privilege mode
tdr-cable-test tengigabitethernet slot/port
Between two ports, do not start the test on both ends of the cable.
Enable the interface before starting the test.
Enable the port to run the test or the test prints an error message.
2. Displays TDR test results.
EXEC Privilege mode
show tdr tengigabitethernet slot/port
Link Dampening
Interface state changes occur when interfaces are administratively brought up or down or if an interface state changes.
Every time an interface changes a state or flaps, routing protocols are notified of the status of the routes that are affected by
the change in state. These protocols go through the momentous task of re-converging. Flapping; therefore, puts the status of
entire network at risk of transient loops and black holes. Dampening limits the notification of status to the routing protocols.
Link dampening minimizes the risk created by flapping by imposing a penalty (1024) for each interface flap and decaying the
penalty exponentially based on the half-time. When the accumulated penalty exceeds a certain threshold (suppress threshold),
the interface is put in an Error-Disabled state and for all practical purposes of routing, the interface is deemed to be “down.”
After the interface becomes stable and the penalty decays below a certain threshold (reuse threshold), the interface comes up
again and the routing protocols re-converge.
You configure link dampening using the dampening [[[[half-life] [reuse-threshold]] [suppress-
threshold]] [max-suppress-time]] command on the interface.
Following is the detailed explanation of interface state change events:
● suppress-threshold— The suppress threshold is a value that triggers a flapping interface to dampen. The system adds
penalty when the interface state goes up and down. When the accumulated penalty reaches the default or configured
suppress threshold, the interface state changes to Error-Disabled state. The range of suppress threshold is from 1 to 20000.
The default is 2500.
● half-life— The accumulated penalty decays exponentially based on the half-life period. The accumulated penalty
decreases half after each half-life period. The range of half-life is from 1 to 30 seconds. The default is 5 seconds.
● reuse-threshold— After exponential decay, the penalty reaches the default or configured reuse threshold. The
interface is unsuppressed and the state changes to “up”. The range of reuse threshold is from 1 to 20000. The default is
750.
● max-suppress-time— The maximum amount of time during which the interface remain suppressed. The range is from 1
to 86400. The default is 20 seconds or four times the default half-life period (5 seconds).
NOTE:
suppress-threshold should be greater than reuse-threshold.
342 Interfaces