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Cisco 819 Series Integrated Services Routers Software Configuration Guide
OL-23590-02
Chapter 8 Configuring the Serial Interface
Information About Configuring Serial Interfaces
Information About Configuring Serial Interfaces
To configure serial interfaces, you must understand the following concept:
• Cisco HDLC Encapsulation, page 8-3
• PPP Encapsulation, page 8-3
• Keepalive Timer, page 8-4
• Frame Relay Encapsulation, page 8-5
Cisco HDLC Encapsulation
Cisco High-Level Data Link Controller (HDLC) is the Cisco proprietary protocol for sending data over
synchronous serial links using HDLC. Cisco HDLC also provides a simple control protocol called Serial
Line Address Resolution Protocol (SLARP) to maintain serial link keepalives. Cisco HDLC is the
default for data encapsulation at Layer 2 (data link) of the Open System Interconnection (OSI) stack for
efficient packet delineation and error control.
Note Cisco HDLC is the default encapsulation type for the serial interfaces.
When the encapsulation on a serial interface is changed from HDLC to any other encapsulation type, the
configured serial subinterfaces on the main interface inherit the newly changed encapsulation and they
do not get deleted.
Cisco HDLC uses keepalives to monitor the link state, as described in the “Keepalive Timer” section on
page 8-4.
PPP Encapsulation
PPP is a standard protocol used to send data over synchronous serial links. PPP also provides a Link
Control Protocol (LCP) for negotiating properties of the link. LCP uses echo requests and responses to
monitor the continuing availability of the link.
Note When an interface is configured with PPP encapsulation, a link is declared down and full LCP
negotiation is re-initiated after five echo request (ECHOREQ) packets are sent without receiving an echo
response (ECHOREP).
PPP provides the following Network Control Protocols (NCPs) for negotiating properties of data
protocols that will run on the link:
• IP Control Protocol (IPCP) to negotiate IP properties
• Multiprotocol Label Switching control processor (MPLSCP) to negotiate MPLS properties
• Cisco Discovery Protocol control processor (CDPCP) to negotiate CDP properties
• IPv6CP to negotiate IP Version 6 (IPv6) properties
• Open Systems Interconnection control processor (OSICP) to negotiate OSI properties