Interface configuration is a common source of physical layer problems.
Figure
shows possible
problems that can cause the line status or the line protocol to be down.
Commonly on serial interfaces lack of physical connectivity could occur
because the interface at the clocking end has not been set with a clock rate,
or it has been set incorrectly. Also the no shutdown
command must be applied to the interface; otherwise it will remain in an
administratively down state, which means that it has effectively been turned
off.
On serial connections using Frame Relay or ISDN equivalent
encapsulation, protocols must be set at both sides of the link. HDLC has many
implementations, with different vendors creating different HDLC versions that
most often are proprietary and not inter-compatible. Cisco has developed its
own implementation of HDLC. If a Cisco device is connected through a serial
link to another vendors equipment it is usual to implement the IETF HDLC on
both devices. This version of HDLC is often used between dissimilar equipment
because it is a common second choice when a proprietary version of HDLC will
not suffice.