PPP Related Configuration in Cisco IOS

PPP (Point-to-Point Protocol) is a WAN protocol which operates in Layer 2 and can be used to run IP, IPv6, IPX, AppleTalk, CDP etc on top of it. It is basically an Encapsulation.

PPP has 2 built in protocols.

1. LCP (Link Control Protocol)
2. NCP (Network Control Protocol)

LCP is the protocol which makes up the link. If authentication is enabled, this is the protocol which takes care of it.
NCP is the protocol which makes other protocols to run upon it.

PPP authentication has 2 types.

1. PAP (Password Authentication Protocol)
2. CHAP (Challenge Handshake Authentication Protocol)

PAP basically send username and password in clear text and CHAP sends a challenge and receive a hash to compare. Both does the same thing, its just CHAP is more secure.

Following is the R1's configuration..

R1(config)#interface serial 1/0
R1(config-if)#encapsulation ppp
R1(config-if)#clock rate 64000
R1(config-if)#ip address 192.168.12.1 255.255.255.0
R1(config-if)#ppp authentication chap

R1(config)#username R2 password PASSWORD

R2 will have the following configuration..

R2(config)#interface serial 1/0
R2(config-if)#encapsulation ppp
R2(config-if)#ip address 192.168.12.2 255.255.255.0
R2(config-if)#ppp authentication chap

R2(config)#username R1 password PASSWORD

Because this is a serial interface, one should be the DCE (which gives the clock rate) and in this example, that is R1.

Username is the host name of the other side and the password should be matched.