What is HDLC?

High-level Data Link Control Protocol (HDLC) is an L2 (data-link layer) network protocol that services point-to-point topologies (e.g. communication protocol on a T1 between a customer premises unit and a telco).

How to configure HDLC

enable
configure terminal
interface gigabit ethernet 1/0/x
ip address %ipv4Address% %subnetMask%
encapsulation hdlc
no shutdown

Alternatively, you can just disable whatever encapsulation %protocol% is currently configured. IOS will default to HDLC.

enable
configure terminal
interface gigabit ethernet 1/0/x
ip address %ipv4Address% %subnetMask%
no encapsulation %protocol%
no shutdown

You can also  use clock rate %speed% to define the clocking rate. This isn’t used much though. It’s only applicable to P2P serial links.