Cisco Certified Expert

Hdlc

High-level data link control is a WAN protocol encapsulation method that allows point-to-point connections between two remote sites. Typically, HDLC is used in a leased-line setup. HDLC is a connectionless protocol that relies on upper layers to recover any frames that have encountered errors across a WAN link. HDLC is the default encapsulation on Cisco serial interfaces.

Cisco routers use HDLC encapsulation, which is proprietary. Cisco added an address field in the HDLC frame, which is not present in the HDLC standard. This field is used by Cisco devices to indicate the type of payload (protocol). Cisco routers use the address field in an

HDLC frame to indicate a payload type, but other routers or manufacturers that implement the HDLC standard do not use the address field. HDLC cannot be used to connect a Cisco router with another vendor.

Figure 2-18 displays the HDLC frame format, which shares a common format with the PPP frame format discussed in the next section.

Figure 2-18 HDLC Frame Format

Field Length in Bytes

12 12 Variable 1 1

Figure 2-18 HDLC Frame Format

12 12 Variable 1 1

SAPI - Service Access Point Identifier C/R - Command/Response EA - Extended Address

TEI - Terminal Endpoint Identifier (All 1s indicate a broadcast.)

SAPI - Service Access Point Identifier C/R - Command/Response EA - Extended Address

TEI - Terminal Endpoint Identifier (All 1s indicate a broadcast.)