The 7705 SAR supports Cisco HDLC, which is an encapsulation protocol for information transfer. Cisco HDLC is a bit-oriented synchronous data-link layer protocol that specifies a data encapsulation method on synchronous serial links using frame characters and checksums.
Cisco HDLC monitors line status on a serial interface by exchanging keepalive request messages with peer network devices. The protocol also allows routers to discover IP addresses of neighbors by exchanging SLARP address-request and address-response messages with peer network devices.
The basic frame structure of a cHDLC frame is shown in Table: cHDLC Information Frame.
Flag |
Address |
Control |
Protocol |
Information |
FCS |
|---|---|---|---|---|---|
0x7E |
0x0F, 0x8F |
0x00 |
0x0800, 0x8035 |
— |
16 or 32 bit |
The fields in the cHDLC frame have the following characteristics:
Address field—supports unicast (0x0F) and broadcast (0x8F) addresses
Control field—always set to 0x00
Protocol field—supports IP (0x0800) and SLARP (0x8035; see SLARP for information about limitations)
Information field—the length can be 0 to 9 kbytes
FCS field—can be 16 or 32 bits. The default is 16 bits for ports with a speed equal to or lower than OC3, and 32 bits for all other ports. The FCS for cHDLC is calculated with the same method and same polynomial as PPP.