Cisco Route : PPP

By default Cisco uses HDLC encapsulation on Serial interfaces. We are going to setup a simple PPP link with Authentication.

R1#show int serial 0/0   

Serial0/0 is up, line protocol is up

  Hardware is GT96K Serial

  Internet address is 10.1.1.1/24

  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation HDLC, loopback not set

  Keepalive set (10 sec)

  Last input 00:00:09, output 00:00:07, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: weighted fair
R2#show int serial 0/0

Serial0/0 is up, line protocol is up

  Hardware is GT96K Serial

  Internet address is 10.1.1.2/24

  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation HDLC, loopback not set

  Keepalive set (10 sec)

  Last input 00:00:05, output 00:00:06, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: weighted fair

We have to change that to PPP encapsulation on both sides, other wise there will be a encapsulation mismatch and the Interface will remain up but the line protocol will be down.

R1(config-if)#encapsulation ?

  frame-relay  Frame Relay networks

  hdlc         Serial HDLC synchronous

  lapb         LAPB (X.25 Level 2)

  ppp          Point-to-Point protocol

  smds         Switched Megabit Data Service (SMDS)

  x25          X.25
R1(config-if)#encapsulation ppp
R1(config-if)#

*Mar  1 00:05:27.739: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down

R1(config-if)#exit                             

R1(config)#exit

R1#sh int serial 0/0

Serial0/0 is up, line protocol is down 

  Hardware is GT96K Serial

  Internet address is 10.1.1.1/24

  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation PPP, LCP Listen, loopback not set
R2(config)#int serial 0/0

R2(config-if)#encapsulation ppp

R2(config-if)#exit

R2(config)#exit

R2#sh int serial 0/0

Serial0/0 is up, line protocol is up 

  Hardware is GT96K Serial

  Internet address is 10.1.1.2/24

  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation PPP, LCP Open

We should move from HDLC to PPP because PPP has some features that HDLC doesn’t for example, Authentication options, error detection and error recovery features.

Password Authentication Protocol (PAP)  and  Challenge Authentication Protocol (CHAP)

PAP is very passive authentication, where as CHAP actively asks who are you?

PAP also sends username and password in Clear Text.

Here is how to configure CHAP on both routers

The username is the Hostname of the Peer Router you are authenticating to. The passwords must match.

R1(config)#username R2 password TPW

R1(config)#int serial 0/0

R1(config-if)#ppp authentication chap

 

R2(config)#username R1 password TPW

R2(config)#int serial 0/0

R2(config-if)#ppp authentication chap

Most likely if there is a issue its with the passwords mismatching, but you can always use the command:

R1#debug ppp authentication

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