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576 Chapter 8: WAN Protocols and Design

CHAP authentication is passed. Figure 8-26 illustrates the message flow in PAP and CHAP
environments. Example 8-25 shows the CHAP configuration for Figure 8-26.

Figure 8-26

PAP and CHAP Messages

Notice that each router refers to the other router's host name; each router uses its own host name
in CHAP flows, unless overridden by configuration. Each codes the same password. When
Router Barney receives a challenge from Router Fred, Router Barney sends an encrypted value,
which is the value Fred should compute given the name Barney, the password Bedrock, and the
original random number. CHAP authentication is completed if the two values match.

Example 8-25

CHAP Configuration Sample

Router Fred Router Barney

username Barney password Bedrock username Fred password Bedrock
! !
Interface serial 0 Interface serial 0
encapsulation ppp encapsulation ppp
ppp authentication chap ppp authentication chap
. .

Waiting

for Dial

Fred

Dialing
Router

Barney

CHAP

Challenge!

I am #$%#$@

Waiting

for Dial

Fred

Dialing
Router

Barney

PAP

I am Barney

Password = Betty

Ack.

ch08.fm Page 576 Monday, March 20, 2000 5:17 PM