PPP Command List

PPP Command List

PPP configuration is often done in connection with configuring other services. To configure PPP on the router, complete the following tasks:

1. Set PPP encapsulation on the interface. You must set the encapsulation method to PPP before you can configure authentication or compression.
2. Select CHAP and/or PAP as the authentication method (optional).
3. If authentication is used, configure username/password combinations.

PPP options are configured in interface mode for a specific interface.

Use . . .	To . . .
Router(config-if)#encapsulation ppp	Set the encapsulation type to PPP
Router(config-if)#ppp authentication <chap|pap> Router(config-if)#ppp authentication chap pap	Set the authentication method(s) When multiple methods are specified, the first method will be tried first
Router(config-if)#ppp compression	Set compression options
Router(config-if)#ppp chap|pap password <password>	Set the password used with CHAP or PAP for an unknown host
Router(config)#username <hostname> password <password>	Set the username and password for the local router
Router(config)#bandwidth <value>	Set a bandwidth value for an interface
Router#show interface	View encapsulation and PPP information on an interface

To hide the CHAP password from view in the configuration file, use the service password-encryption command from the global configuration mode.

Example
The following commands configure the SFO router to use PPP and enable it to connect to the LAX router using PAP authentication.

SFO(config)#hostname LAX password cisco5

SFO(config)#int s0/1/0

SFO(config-if)#encap ppp

SFO(config-if)#ppp auth pap

PPP Facts

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.