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.
| Flag | Address | Control | Protocol | Information field | FCS |
|---|---|---|---|---|---|
0x7E |
0x0F/0x8F |
0x00 |
— |
— |
16/32 bits |
Address field — The values of the address field include: 0x0F (unicast), 0x8F (broadcast).
Control field — The control field is always set to value 0x00.
Protocol field — The following values are supported for the protocol field:
Table 2 shows the cHDLC protocol fields.
| Protocol | Field value |
|---|---|
IP |
0x0800 |
Cisco SLARP |
0x8035 |
ISO CLNP/ISO ES-IS DSAP/SSAP1 |
0xFEFE |
Information field — The length of the information field is in the range of 0 to 9 kbytes.
FCS field — The FCS field can assume a 16-bit or 32-bit value. The default is 16-bits for ports with a speed equal to or lower than OC-3, and 32-bits for all other ports. The FCS for cHDLC is calculated in the same manner and same polynomial as PPP.