Ccna routing and Switching Complete Study Guide

LCP Closed

  

Closed: IPCP, CDPCP

First, notice that the first line of output shows us that 

Serial0/0 is up

 and 

line protocol 

is down

. This is because there are no keepalives coming from the remote router. The next thing 

I want you to notice is that the LCP and NCP are closed because the authentication failed.

Debugging PPP Authentication

To display the CHAP authentication process as it occurs between two routers in the 

network, just use the command 

debug ppp authentication

.

If your PPP encapsulation and authentication are set up correctly on both routers and your 

usernames and passwords are all good, then the 

debug ppp authentication

 command will 

display an output that looks like the following output, which is called the three-way handshake:

d16h: Se0/0 PPP: Using default call direction

1d16h: Se0/0 PPP: Treating connection as a dedicated line

1d16h: Se0/0 CHAP: O CHALLENGE id 219 len 27 from "Pod1R1"

1d16h: Se0/0 CHAP: I CHALLENGE id 208 len 27 from "Pod1R2"

1d16h: Se0/0 CHAP: O RESPONSE id 208 len 27 from "Pod1R1"

1d16h: Se0/0 CHAP: I RESPONSE id 219 len 27 from "Pod1R2"

1d16h: Se0/0 CHAP: O SUCCESS id 219 len 4

1d16h: Se0/0 CHAP: I SUCCESS id 208 len 4

But if you have the password wrong as they were previously in the PPP authentication 

failure example back in Figure 21.15, the output would look something like this:

1d16h: Se0/0 PPP: Using default call direction

1d16h: Se0/0 PPP: Treating connection as a dedicated line

1d16h: %SYS-5-CONFIG_I: Configured from console by console

1d16h: Se0/0 CHAP: O CHALLENGE id 220 len 27 from "Pod1R1"

1d16h: Se0/0 CHAP: I CHALLENGE id 209 len 27 from "Pod1R2"

1d16h: Se0/0 CHAP: O RESPONSE id 209 len 27 from "Pod1R1"

1d16h: Se0/0 CHAP: I RESPONSE id 220 len 27 from "Pod1R2"

1d16h: Se0/0 CHAP: O FAILURE id 220 len 25 msg is "MD/DES compare failed"

PPP with CHAP authentication is a three-way authentication, and if the username and 

passwords aren’t configured exactly the way they should be, then the authentication will 

fail and the link will go down.

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Point-to-Point Protocol (PPP)