PPP , HDLC

As you know there are some wan protocols that you can use them on wan .
PPP , HDLC , Frame Relay , ATM

PPP and HDLC are 2 protocols that used in leased line .

PPP have some advantages than HDLC . PPP is a data link protocol that :

1. Definition of header and trailer can transport layer 3 protocol on a same link .
2. Builts in Authentication protocol (PAP and CHAP)
3. Have LCP (Link Control Protocol ) in frame.
4. PPP compression
   

with LCP , PPP have most ability to control the links :

• Loop Detection : with magic number , that each router is used
  
• Error Detection : With FCS in frame and use LQM (Link Quality Monitoring )
• Authentication : PAP & CHAP
• Multi Link support : (Load balancing )

Authentication :

• PAP (Powered Authentication Protocol) : Send password as clear Text .
• CHAP (Challenge Authentication Protocol ) : More secure then PPP because of use MD5 one way hash .

for Authentication configuration :

1. use Hostname for end link router .
2. use this command on each router : # username The other end link router-name password which is same on both router from example :
   R1(config)#username R2 password mypassword
3. and this command : # PPP authentication PAP or PAP CHAP

Troubleshooting PPP :

1- Layer 1 problems : (when line status down , line protocol is down )

• Physically problem
  
• CSU/DSUhas failed or is misconfigured
  
• Link may be not pluged in to CSU/DSU from telco

2-layer 2 problems : (When line status Up , line protocol is down )

• Clock Rate not set
  
• mismatch Encapsulation or Encapsulation not set
  
• misconfiguration Authentication
• keepalive is not set

3- layer 3 problems : (When line status Up , line protocol is Up )

• mismatched IP subnet (link work if encapsulation is PPP , but if encapsulation is HDLC link is not work )

HDLC :

• the other wan protocol that is a default encapsulation used by cisco router .
• No authentication can be used with HDLC .
• PPP is a protocol that can use with both asynchronous(Dial-up) and synchronous (ISDN).