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.