Configuring HDLC

Think back to router LAN interfaces for a moment. Routers require no configuration related to Layers 1 and 2 for the interface to be up and working, forwarding IP traffic. The Layer 1 details occur by default once the cabling has been installed correctly. Router Ethernet interfaces of course use Ethernet as the data link protocol by default. The router only needs to configure an IP address on the interface, and possibly enable the interface with the no shutdown command if the interface is in an “administratively down” state.

Similarly, serial interfaces on Cisco routers need no specific Layer 1 or 2 configuration commands. For Layer 1, the cabling needs to be completed, of course, but the router attempts to use the serial interface ...

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