Cisco Systems OL-6426-02 Benutzerhandbuch

(start/stop) and bit-oriented synchronous encapsulation, network protocol multiplexing, link 
configuration, link quality testing, error detection, and option negotiation for such capabilities as 
network-layer address negotiation and data-compression negotiation. PPP supports these functions by 
providing an extensible Link Control Protocol (LCP) and a family of Network Control Protocols (NCPs) 
to negotiate optional configuration parameters and facilities.

The current implementation of PPP supports two security authentication protocols to authenticate a PPP 
session:

Password Authentication Protocol (PAP)

Challenge Handshake Authentication Protocol (CHAP)

PPP with PAP or CHAP authentication is often used to inform the central site which remote routers are 
connected to it. 

PAP uses a two-way handshake to verify the passwords between routers. To illustrate how PAP works, 
imagine a network topology in which a remote office Cisco router is connected to a corporate office 
Cisco router. After the PPP link is established, the remote office router repeatedly sends a configured 
username and password until the corporate office router accepts the authentication. 

PAP has the following characteristics:

The password portion of the authentication is sent across the link in clear text (not scrambled or 
encrypted). 

PAP provides no protection from playback or repeated trial-and-error attacks. 

The remote office router controls the frequency and timing of the authentication attempts.

CHAP uses a three-way handshake to verify passwords. To illustrate how CHAP works, imagine a 
network topology in which a remote office Cisco router is connected to a corporate office Cisco router. 

After the PPP link is established, the corporate office router sends a challenge message to the remote 
office router. The remote office router responds with a variable value. The corporate office router checks 
the response against its own calculation of the value. If the values match, the corporate office router 
accepts the authentication. The authentication process can be repeated any time after the link is 
established.

CHAP has the following characteristics:

The authentication process uses a variable challenge value rather than a password.

CHAP protects against playback attack through the use of the variable challenge value, which is 
unique and unpredictable. Repeated challenges limit the time of exposure to any single attack.

The corporate office router controls the frequency and timing of the authentication attempts.

We recommend using CHAP because it is the more secure of the two protocols.