CCNA Cisco Certified Network Associate Study Guide - FTP Server

Point-to-Point Protocol (PPP) 485
Authentication phase If configured, either CHAP or PAP can be used to
authenticate a link. Authentication takes place before Network-layer protocol
information is read.
Network-layer protocol phase PPP uses the Network Control Protocol
to allow multiple Network-layer protocols to be encapsulated and sent
over a PPP data link.
PPP Authentication Methods
There are two methods of authentication that can be used with PPP links,
either Password Authentication Protocol (PAP) or Challenge Authentication
Protocol (CHAP).
Password Authentication Protocol (PAP)
The Password Authentication Protocol (PAP) is the less secure of the two
methods. Passwords are sent in clear text, and PAP is only performed upon
the initial link establishment. When the PPP link is first established, the
remote node sends back to the sending router the username and password
until authentication is acknowledged. That’s it.
Challenge Authentication Protocol (CHAP)
The Challenge Authentication Protocol (CHAP) is used at the initial startup
of a link and at periodic checkups on the link to make sure the router is still
communicating with the same host.
After PPP finishes its initial phase, the local router sends a challenge
request to the remote device. The remote device sends a value calculated
using a one-way hash function called MD5. The local router checks this hash
value to make sure it matches. If the values don’t match, the link is immediately
terminated.
Configuring PPP on Cisco Routers
Configuring PPP encapsulation on an interface is a fairly straightforward
process. To configure it, follow these router commands:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int s0