CCNA

Configuring PPP Encapsulation

Jay

17 Jul 2009 • 1 min read

Point-to-Point Protocol (PPP) is a data link layer protocol that is used to establish a direct connection between two network nodes. It is commonly used for serial connections, dial-up connections, and VPNs. PPP provides features such as authentication, compression, and error detection.

In this article, we'll learn how to configure PPP encapsulation on Cisco routers.

Basic PPP Configuration

To configure PPP encapsulation on a serial interface, use the following commands:

Router(config)# interface serial 0/0/0
Router(config-if)# encapsulation ppp
Router(config-if)# no shutdown

This configures the serial interface to use PPP encapsulation instead of the default HDLC encapsulation.

PPP Authentication

PPP supports two authentication protocols:

PAP (Password Authentication Protocol) - Sends passwords in clear text
CHAP (Challenge Handshake Authentication Protocol) - Uses a three-way handshake and MD5 hashing

Configuring PAP Authentication

To configure PAP authentication:

Router(config)# username remote-router password cisco
Router(config)# interface serial 0/0/0
Router(config-if)# encapsulation ppp
Router(config-if)# ppp authentication pap
Router(config-if)# ppp pap sent-username local-router password cisco

Configuring CHAP Authentication

To configure CHAP authentication:

Router(config)# username remote-router password cisco
Router(config)# interface serial 0/0/0
Router(config-if)# encapsulation ppp
Router(config-if)# ppp authentication chap

Verification Commands

Use these commands to verify PPP configuration:

Router# show interfaces serial 0/0/0
Router# show ppp all
Router# debug ppp negotiation
Router# debug ppp authentication

Troubleshooting PPP

Common PPP issues include:

Authentication failures - Check username/password configuration
Encapsulation mismatch - Ensure both sides use PPP
Interface down - Check physical connectivity and no shutdown command

PPP encapsulation is essential for establishing reliable point-to-point connections in enterprise networks. Understanding its configuration and troubleshooting is crucial for network administrators.