Serial framing and compatible encapsulation
HDLC defines bit-oriented data-link framing and procedures. Framing lets a receiver identify where each frame starts and ends, and a frame check sequence lets it detect corruption. These procedures appear in older synchronous serial WANs.
Cisco HDLC is a vendor-specific serial encapsulation that uses keepalives to monitor the peer. A matching physical interface does not guarantee compatible framing. When connecting different equipment, confirm that both ends support the same encapsulation. PPP offers a standardized alternative with explicit link and network protocol negotiation.
Try it in a lab
- On two compatible serial lab interfaces, configure Cisco HDLC on both ends and verify that the line protocol and test traffic come up.
- Change one end to PPP. Compare physical and line protocol status and verify that traffic fails. Match the encapsulation on both ends again and confirm recovery.