Cisco HDLC

Cisco HDLC (cHDLC) is an encapsulation protocol for information transfer. It is a bit-oriented synchronous data-link layer protocol that specifies a data encapsulation method on synchronous serial links using frame characters and checksums.

cHDLC monitors line status on a serial interface by exchanging keepalive request messages with peer network devices. It also allows routers to discover IP addresses of neighbors by exchanging Serial Link Address Resolution Protocol (SLARP) (see SLARP) address-request and address-response messages with peer network devices.

The basic frame structure of a cHDLC frame is shown in Table 1. This frame structure is similar to PPP in an HDLC-link frame (RFC 1662, PPP in HDLC-like Framing). The differences to PPP in and HDLC-like frames are in the values used in the address, control, and protocol fields.

Table 1. cHDLC I-Frame

Flag

Address

Control

Protocol

Information Field

FCS

0x7E

0x0F/0x8F

0x00

16/32 bits