Cisco IOS Legacy WAN: HDLC, PPP, Frame Relay, and PPPoA
Working reference for legacy Cisco WAN encapsulations. HDLC as the Cisco-only proprietary default with no authentication, PPP as the standards-based alternative with PAP and CHAP authentication (cross-username pattern, challenge-response hash for CHAP), PPP compression and multilink, PPPoA for DSL access. Frame Relay configuration with DLCI / LMI / map statements, the multipoint vs point-to-point sub-interface decision (sub-interfaces sidestep split-horizon issues), Frame Relay verification commands. Pitfalls: HDLC mismatch with non-Cisco peers, CHAP password mismatch, Frame Relay split-horizon on multipoint, missing Inverse ARP, DCE clocking on lab serials, LMI type mismatch.