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Tuesday, November 2, 2010

Clockrate, DTE/DCE -Why are we unable to remove clock rate?

We are currently preparing a new 1841 with a HWIC-1T serial interface. We need to pre configure the router which will then be shipped to where it is going to be used. The router currently does not have a serial connection attached to it (as we don't have a serial connection). We've reached the configuration of the serial interface and we are having some problems removing the "clock rate 2000000" command to make it a DTE.We tried the following commands:

no clock rate

no clock rate 2000000

default clock rate

We already have a router onsite with a working serial interface and that one does not have any clock rate set.

Tips:

The clock rate command is added in newer IOS versions automatically to the interface and cannot be removed manually. It will be automatically removed if you connect the DTE cable to the serial interface. So don't worry about that command being present in the configuration right now. It will disappear automatically when a DTE cable is plugged to the port.

Note that a serial interface cannot be configured to work in Layer1 DCE or DTE mode by command. This depends exclusively on the type of the cable. Most certainly, the clock rate command simply sets the clockrate to be used if a DCE-type cable is connected. Otherwise, that command is ignored.

In older IOSes, the command could actually be removed from the interface but that was a source of common configuration mistake - both routers connected, interfaces went up but line protocol remained down because the clock rate was not set and thus data could not be carried through the synchronous serial link. Therefore, newer IOSes add the clock rate setting automatically (set to a sensible default).

So to put it short - if the router is going to be the DTE, simply ignore the clock rate command.

Citation - This blog post does not reflect original content from the author. Rather it summarizes content that are relevant to the topic from different sources in the web. The sources might include any online discussion boards, forums, websites and others.

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