Cisco-PPP

		PPP
	Internetworking Technologies Instructor: Prabul, CCNA

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10. Point-to-Point Protocol (PPP)

Top 10 Graphics:

WAN connections are controlled by 'protocols' that perform the same basic functions as layer 2 LAN protocols:

Establish a data path
Use flow control procedures to assure delivery of data

Point to Point Protocol (PPP) is the most widely used Wan protocol and performs the following functions:

Link establishment - This function opens a connection or path between two routers and negotiates how data will be sent across this network path
Link quality determination - This function will test the link to make sure that the data path is stable and reliable
Network layer protocol configuration - This function establishes which layer 3 protocols will be sent across the data path since PPP can deliver multiple layer 3 protocols such as IP and IPX at the same time both sides of the connection need to know what protocols will be sent.
Link termination - This function will terminate the WAN data path between two routers.

PPP Overview

PPP provides reliable connections between routers
PPP is a data link with network layer services
PPP provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits
Created to solve remote Internet connectivity problems
PPP can cary packets from several protocol suites using NCPs
PPP controls the setup of several link options using LCP

NCP (Network Control Programs) - Functional fields containing standardized codes to indicate the network-layer protocol type that PPP encapsulates.

LCP (Link Control Protocol) - is used to establish, configure, and test the data link connection.

PPP Operation

PPP runs on the following types of WAN physical interfaces:
1. Asynchronous serial
2. ISDN
3. Synchronous serial
PPP datagram transmission employs "three key components" to provide effective data transmission:
1. Encapsulation - PPP supports the High-Level Data Link Control (HDLC) protocol to provide encapsulation.
2. Link Control Protocol (LCP) - An extendible LCP is used to establish, configure, and test the data link connection.
3. Network Control Protocols (NCPs) - a family of NCPs are used to establish and configure different network layer protocols.

PPP connections are established in stages:

An originating PPP node first sends LCP frames to configure and test the data link.
The link is established, and facilities are negotiated
The originating PPP node then sends NCP frames to choose and configure network layer protocols.
The chosen network layer protocols (TCP/IP, Novell IPX, AppleTalk) are configured, and packets from each network layer protocol are sent.
The link available open until LCP or NCP frames close the link or until some external event occurs.

PPP Frame Format:

Bytes:    1      1        1        2       var    2|4
Field: [Flag][Address][Control][Protocol][ Data ][FCS]

Flag - Indicates the beginning or end of a frame and consists of the binary sequence 01111110.

Address - Consists of the standard brodcast address, binary sequence 11111111. PPP does not assign individual station addresses.

Control - 1 byte that consists of the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. A connectionless link service similar to that of Logical Link Control (LLC) Type 1 is provided.

Protocol - 2 bytes that identify the protocol encapsulated in the Information field of the frame. The most up-to-date values of the Protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).

Data - Zero or more bytes that contain the datagram for the protocol specified in the Protocol field. The end of the Information field is found by locating the closing flag sequence and allowing 2 bytes for the FCS field. The default maximum length of the Information field is 1,500 bytes. By prior agreement, consenting PPP implementations can use other values for the maximum Information field length.

Frame Check Sequence (FCS) - Normally 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection.

Note: The PPP Link Control Protocol (LCP) can negotiate modifications to the standard PPP frame structure. However, modified frames will be clearly distinguishable from standard frames.

PPP LCP (Link Control Protocol) Configuration options:

Authentication (Protocol: PAP or CHAP)
- Require a password
- Perform Challenge Handshake
Compression (Protocol: Stacker or Predictor)
- Compress data st source;
  reproduce data at destination
Error Detection (Protocol: Quality & Magic Number)
- Monitor data dropped on link
- Avoid frame looping
Multilink (Protocol: Multilink PPP)
- Load balancing across multiple links

PPP Link Control Protocol (LCP) goes through four distinct phases:

Link establishment and configuration negotiation
(phase is complete when a configuration acknowledgment frame has been sent and received)
(Optional) Link quality determination
(link is tested to determine whether the quality is sufficient to bring up network layer protocols)
Network layer protocol configuration negotation
(network layer protocols can be separately configured by the appropriate NCP)
Link termination
(by request of a user or due to physical event)

A PPP Session establishment has three phases:

Link Establishment Phase
- each PPP device sends LCP packets to configure/test the data link
- LCP packets contain a Configuration Option field to negotiate:
  - maximum receive unit
  - compression of certain PPP fields
  - link authentication protocol
(Optional) Authentication Phase
- PAP - Password Authentication Protocol
  - Two-way handshake
  - Passwords sent in clear text
  - Remote node in control of attempts
- CHAP - Challenge Handshake Authentication Protocol
  - Three-way handshake
  - Challenge | Response | Accept/Reject
  - Use secret know only to authenticator and peer
  - Can be repeated any time after the link has been established
Network-Layer Protocol Phase
- PPP devices send NCP packets to choose and configure one or more network-layer protocols
- Once protocols are configured, datagrams can be sent over the network

Configuring PPP with Authentication:

Authenticating Router (The router that receives the call)
- host name
- username
- ppp authentication
Router to be Authenticated (The router that initiates the call)
- hostname
- username
- ppp authentication
- ppp pap sent-username (PAP only)


Router(config)# username 'name' password 'secret'

The host name of the remote router (case sensitive)
secret password must be the same for both routers

Note: To enable the local router to respond to remote CHAP challenges, one username name entry must be the same as the host name name entry that has already been assigned to your device.


Router(config)# encapsulation ppp

Configure the interface for PPP encapsulation


Router(config-if)# ppp pap sent-username 'username' 
                   password 'password'

Enable PAP


Router(config-if)# ppp chap hostname 'hostname'

Use the same host name on multiple routers


Router(config-if)# ppp chap password 'secret'

Use a password to authenticate to an unknown host


Router(config-if)# ppp authentication 
                   {chap | chap pap | pap chap | pap}

Configure PPP authentication


Router# show interfaces S1

When PPP is configured, you can check its LCP and NCP states using the 'show interfaces' command

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This page is maintained by: Prabul

			`aprabul@yahoo.com` `Copyright © 2003`