Cable Guy wrote: > > When you put a hardware loopback plug on a serial to test it, why does the > router see it as a loopback? With the show int serial command, you will see > -----serial up, line protocol up (loopback mode)----- > Of course it needs to see it as loopback otherwise the router wouldn't > operate and allow loopback tests. But what part of the digital signals > specifically tell the router that what it sent out has just come back? It > can't be the carrier detect because that could be interpreted as a carrier > detect from any csu/dsu. Maybe a timing issue on the return signal, but > sounds like the keepalives are being interpreted here somehow? Someone > know the details of this? Help. > Check the archives on the topic of HDLC keepalives. They have sequence numbers as well as a (layer 2) DTE/DCE flag. Experiment with "debug serial interface". - Marty
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