Port Features
Page 68 7750 SR Interface Configuration Guide
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 on page 69) address-request and
address-response messages with peer network devices.
The basic frame structure of a cHDLC frame is shown in Table 14. 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.
• Address field — The values of the address field include: 0x0F (unicast), 0x8F (broadcast).
• Control field — The control field is a
lways set to value 0x00.
• Protocol field — The following values are supported for the protocol field:
• Information field — The length of the information field is in the range of 0 to 9Kbytes.
• 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.
Table 14: cHDLC I-Frame
Flag Address Control Protocol
Information
Field
FCS
0x7E 0x0F/0x8F 0x00 — — 16/32 bits
Table 15: cHDLC Protocol Fields
Protocol Field Value
IP 0x0800
Cisco SLARP 0x8035
ISO CLNP/ISO ES-IS DSAP/SSAP1 0xFEFE
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