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 Education Made Easy

What is PPP and why we use it?

In computer networking, Point-to-Point Protocol (PPP) is a data link protocol used to establish a direct connection between two nodes. PPP is specified in RFC 1661.

Functionality: Similar to link layer services (encapsulate L3) + Extra management functions

PPP (Point-to-Point Protocol) is a protocol for communication between two computers using a serial interface, typically a personal computer connected by phone line to a server. For example, your Internet server provider may provide you with a PPP connection so that the provider's server can respond to your requests, pass them on to the Internet, and forward your requested Internet responses back to you. PPP uses the Internet protocol (IP) (and is designed to handle others).

PPP is comprised of three main components:

A method for encapsulating multi-protocol datagrams.

A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.

A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.

What are the PPP Features?

Protocol Multiplexing : via Multiple NCPs

PPP can detect network layer signal failure also and learn network layer address

Authentication (optional): via PAP/CHAP

Compression (optional): Stacker/Predictor

Loopback detection: Via LCP using magic no. of each router

Load balancing: MLPPP (multilink ppp)

Not in PPP: Error recovery, flow control, data re-ordering all relegated to higher layers!

What are physical link require for PPP?

serial cable

phone line

trunk line

cellular telephone

specialized radio line

fiber optic links such as SONET

Encapsulation :             (See the below figure)

PPP uses HDLC?

PPP uses the HDLC Frame Check Sequence for error detection

How to configure PPP?

Router#configure terminal

Router(config)#interface serial 0/0/0

Router(config-if)#encapsulation ppp


Router#show interfaces serial 0/0/0

Serial0/0/0 is up, line protocol is down (disabled)

Hardware is HD64570

Internet address is

MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Encapsulation PPP, loopback not set, keepalive set (10 sec)

 [output is omited]

How to configure Chap Authentication?


Router#configure terminal

Router(config)#hostname R1

R1(config)#username R2 password anukeer

R1(config)#interface serial 0/0/0

R1(config-if)#encapsulation ppp

R1(config-if)#ppp authentication chap



What to do when data byte looks like flag byte in the middle?

One byte stuffing (01111101 )is done before such data byte as:

== B1 C2 01111110 01111101 A8 ==>

How PPP is preferred over SLIP?

PPP is usually preferred over the earlier de facto standard Serial Line Internet Protocol (SLIP) because it can handle synchronous as well as asynchronous communication. PPP can share a line with other users and it has error detection that SLIP lacks.

PPP Class Notes

 Link Control Protocol (LCP)

Authentication protocols

o Password Authentication Protocol (PAP)

o Challenge Handshake Authentication Protocol (CHAP)

Network Control Protocols (NCP) :IPCP/CCP

o Internet Protocol Control Protocol (IPCP): used when IP data carry

o Compression Control Protocol (CCP)