PPP stands for Point-to-Point and is a protocol operating at Layer 2 of the OSI model.
HDLC is Cisco proprietary and is the default encapsulation on serial links great if you have all Cisco devices, otherwise you will need PPP configured.
There are two types of authentication that can be used with PPP. These are PAP and CHAP.
PAP (Password Authentication Protocol) – Passwords are sent in plain text. No periodic checks.
CHAP (Challenge Handshake Authentication Protocol) – Password is encrypted. Sends periodic checks to ensure the router is talking to the same router.
Configuring PPP is rather simple:
configure terminal int s2/0 encapsulation ppp
Configure PPP with PAP authentication
RouterA:
configure terminal hostname RouterA username RouterB password Cisco int s2/0 ip address 10.0.0.1 255.255.255.0 encapsulation ppp ppp authentication pap ppp pap sent-username RouterA password Cisco
RouterB:
configure terminal hostname RouterB username RouterA password Cisco int s2/0 ip address 10.0.0.2 255.255.255.0 encapsulation ppp ppp pap sent-username RouterB password Cisco ppp authentication pap
Configure PPP with CHAP authentication
RouterA:
configure terminal hostname RouterA username RouterB password Cisco int s2/0 ip address 10.0.0.1 255.255.255.0 encapsulation ppp ppp authentication chap
RouterB:
configure terminal hostname RouterB username RouterA password Cisco int s2/0 ip address 10.0.0.2 255.255.255.0 encapsulation ppp ppp authentication chap
Show commands:
show interface s0/0 – Look for encapsulation ppp and LCP is open. Check interface is up and line protocol is up. Password/Username or encapsulation mismatch will bring the line protocol down.
Debug:
debug ppp authentication – Look for any failures of the three-way handshake.