# Overview

Cisco HDLC is a bit-oriented synchronous data-link layer protocol developed by the International Organization for Standardization (ISO). It specifies a data encapsulation method on synchronous serial links using frame characters and checksums.

By default, synchronous serial lines use the HDLC serial encapsulation method, which provides the synchronous framing-detection and error-detection functions of HDLC without windowing or retransmission.

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

The router responds to a SLARP address-request message from a remote peer with a SLARP address-response message, which indicates that it cannot participate in a SLARP session.

Cisco HDLC is compatible with Cisco Systems Cisco-HDLC protocol, the default protocol for all Cisco serial interfaces.

## Framing

The router supports the following framing features:

- HDLC for data-link framing
- 18,000-byte information field size

## Error Frames

All Cisco HDLC error frames are discarded.

## SLARP Keepalive

One feature of Cisco HDLC is the exchange of keepalive messages. A keepalive message is a signal from one endpoint to the other that the first endpoint is still active. Keepalives are used to identify inactive or failed connections.