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Catalyst 2950 and Catalyst 2955 Switch Command Reference
OL-10102-01
Chapter 2 Catalyst 2950 and 2955 Cisco IOS Commands
margin
margin
Use the margin interface configuration command to specify the margin value used to determine link
quality during Long-Reach Ethernet (LRE) rate selection. Use the no form of this command to return to
the default setting.
margin {downstream value | upstream value}
no margin {downstream value | upstream value}
This command is available only on Catalyst 2950 LRE switches.
Syntax Description
Defaults The default is 0 dB.
Command Modes Interface configuration
Command History
Usage Guidelines When rate selection is running, the signal-to-noise ratio (SNR) shows the link quality. The switch does
not provide any internal mechanism to ensure link quality. There can be different requirements for link
quality, depending on the required bit-error rate and the noise level of the environment. A noisier
environment would require a higher SNR in order to provide a stable link. A lower bit-error rate would
require a higher SNR. Typically a 6-dB margin provides an error rate of 10
-21
bits.
To provide link stability, you should add a margin to the required SNR. You can configure your margins
to an amount that is appropriate for the noise level of your environment. Increasing the margin
requirement can cause the system to choose a lower profile, which would in turn translate to a lower rate
but with a longer reach.
The switch does not guarantee any margins after a link is activated. Margins are only guaranteed when
the link is established. When a link is activated, if the SNR requirements do not match the configured
margin level, the link is not established.
Downstream means the remote end of the link and upstream the local end. The link has to satisfy both
the local and remote margin requirements. If either one is not met, the link is advertised as down. This
command has no significance if rate selection is not running on this interface.
Note Setting a margin requirement might cause the system to choose another profile, which might translate to
a lower rate but a longer reach.
downstream value Value at the remote end of the link. The range is 0 to 10 decibels (dB).
upstream value Value at the local end of the link. The range is 0 to 10 dB.
Release Modification
12.1(11)YJ This command was introduced.
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