Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP) Facts

The following list represents some of the key features of the Point-to-Point Protocol (PPP):

• It can be used on a wide variety of physical interfaces including asynchronous serial, synchronous serial (dial up), and ISDN.
• It supports multiple Network layer protocols, including IP, IPX, AppleTalk, and numerous others.
• Optional authentication is provided through PAP (2-way authentication) or CHAP (3-way authentication).
• It supports multilink connections, load-balancing traffic over multiple physical links.
• It includes Link Quality Monitoring (LQM) which can detect link errors and automatically terminate links with excessive errors.
• It includes looped link detection that can identify when messages sent from a router are looped back to that router. This is done through routers sending magic numbers in communications. If a router receives a packet with its own magic number, the link is looped.

PPP uses two main protocols to establish and maintain the link.

Protocol	Description
Link Control Protocol (LCP)	The Link Control Protocol (LCP) is responsible for establishing, maintaining, and tearing down the PPP link. LCP packets are exchanged periodically to do the following: During link establishment, LCPs are used to agree upon encapsulation, packet size, and compression settings. LCPs also indicate whether authentication should be used. Throughout the session, LCPs are exchanged to detect and correct errors or to control the use of multiple links (multilink). When the session is terminated, LCPs are responsible for tearing down the link. A single Link Control Protocol runs for each physical connection.
Network Control Protocol (NCP)	The Network Control Protocol (NCP) is used to agree upon and configure Network layer protocols to use (such as IP, IPX, or AppleTalk). Each Network layer protocol has a corresponding control protocol packet. Examples of control protocols include: IP Control Protocol (IPCP) CDP Control Protocol (CDPCP) IPX Control Protocol (IPXCP) AppleTalk Control Protocol (ATCP) A single PPP link can run multiple control protocols, one for each Network-layer protocol supported on the link.

PPP establishes communication in three phases.

1. LCP phase. LCPs are exchanged to open the link and agree upon link settings such as encapsulation, packet size, and whether authentication will be used.
2. Authenticate phase (optional). During this phase, authentication-specific packets are exchanged to configure authentication parameters and authenticate the devices. LCPs might also be exchanged during this phase to maintain the link.
3. NCP phase. NCPs are exchanged to agree on upper-layer protocols to use. For example, routers might exchange IPCP and CDPCP packets to agree upon using IP and CDP for Network-layer communications. During this phase, LCPs might continue to be exchanged.

 

 

Credit: Testout 640-802 CCNA Notes