What is PPP?
A point-to-point connection is a data link layer (Layer 2) communication protocol used to establish a direct connection. PPP connections are used to connect LANs to service provider WANs or LAN segments within an organization network.
A LAN-to-WAN point-to-point connection is also referred to as a serial connection or leased-line connection because the lines are leased from a carrier (usually a telephone company) and are dedicated for use by the company leasing the lines.
When you connect to your Internet Service Provider (ISP) through a modem, you establish a connection. This connection between the ISP and your network forms two points on the network. The protocol used for this connectivity is the Point-to-Point Protocol (PPP).
Note: When connecting two Cisco routers, the default serial encapsulation method is HDLC. This implies that Cisco HDLC is compatible only with other Cisco devices. However, when connecting to a non-Cisco router, PPP encapsulation should be used.
At this stage, the main purpose of PPP is to transmit layer-3 packets across a Data Link layer point-to-point link. This is one of the many benefits of using PPP, as it is not proprietary.
PPP can be used over twisted pair, fiber-optic lines, and satellite transmission. PPP provides transport over ATM, Frame Relay, ISDN, and optical links. For security, PPP allows you to authenticate or secure connections using either Password Authentication Protocol (PAP) or the more effective Challenge Handshake Authentication Protocol (CHAP).
Components of PPP:
Frame Format: PPP frames encapsulate network layer protocol packets. A PPP frame includes fields like a frame delimiter, address, control, protocol, data, and a frame check sequence.
Data Encapsulations: this is a method used to encapsulate multi-protocol datagrams. Different network-layer protocols are simultaneously transported and encapsulated over the same link, the flexibility of the PPP design enables it to be compatible with most supporting network devices.
HDLC: A method for encapsulating datagrams over serial links. In HDLC, data is framed with a header and trailer containing control information to ensure reliable transmission. The protocol supports both connection-oriented and connectionless communication, making it versatile for various network configurations. Additionally, HDLC offers error detection and correction mechanisms to maintain data integrity during transmission..
Link Control Protocol: The LCP is used to establish, configure, and test the data link connection. It’s flexible in handling different sizes of packets, detecting a looped-back link, configuration errors, and terminating the link.
Network Control Protocol: NCP is used for establishing and configuring different Network layer protocols. PPP enables the simultaneous use of multiple Network layer protocols. Some of the more familiar NPCs are:
i. Internet Protocol Control Protocol (IPCP)
ii. AppleTalk Control Protocol
iii. Novell IPX Control Protocol
iv. Cisco Systems Control Protocol
v. SNA Control Protocol
vi. Compression Control Protocol.
Summary of Establishing a Point-to-Point WAN Connection with PPP
■ PPP is a common Layer 2 protocol for the WAN. Two components of PPP exist: LCP negotiates the connection and NCP encapsulates traffic.
■ You can configure PPP to use PAP or CHAP. PAP sends everything in plain text. CHAP uses an MD5 hash.
■ Common PPP verification commands include a show interface to verify PPP encapsulation and debug ppp negotiation to verify the LCP handshake.
