Problem with ppp authentication chap

[Searon]

Dear Cisco Community,

It's weird but the moment I do ppp authentication chap I get that my serial link goes down, I have no idea how to fix it none of the solutions I find on google work...

%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/0, changed state to down

ip adresses are set correct aswell

Anyone have an idea what I forget?

ip route 0.0.0.0 0.0.0.0 193.190.239.1

brussel(config)#username brussel password cisco

brussel(config)#

brussel(config)#int s0/0/0

brussel(config-if)#

brussel(config-if)#enc

brussel(config-if)#encapsulation ppp

brussel(config-if)#ppp authentication chap

ip route 0.0.0.0 0.0.0.0 193.190.239.2

isp-1(config)#username isp-1 password cisco

isp-1(config)#

isp-1(config)#int s0/0/0

isp-1(config-if)#encapsulation ppp

isp-1(config-if)#ppp authentication chap

Also hostname is same as the username

[Georg Pauwen · Accepted solution]

Hello,

you need to configure the username of the peer. Make sure the configs look like below:

brussel

username isp-1 password cisco
!
int s0/0/0
encapsulation ppp
clock rate 64000
ppp authentication chap

isp-1

username brussel password cisco
!
int s0/0/0
encapsulation ppp
ppp authentication chap

[Searon]

I tried but the moment I give in "ppp authentication chap" the interface goes down and configuring it on the other router doesn't bring it up like it should..

[Georg Pauwen]

Hello,

reload both routers and post the output of 'sh run'. From the output you have posted, it looks like on the isp-1 router you have entered the username twice, with different passwords ?

[Giuseppe Larosa]

Hello @Searon ,

be very careful when you type the username commands.

We see from your screenshot that you type twice with the first time the password is cisco and the second time is "cisco !" ?

The two passwords must match.

You can use the following commands to debug

debug ppp negotiation

debug ppp authentication

terminal monitor

However, you may have an even more basic issue at OSI layer 1 because the router on the left does not accept the clock rate command and the router on the right has no clock rate command .

But this can be not an issue in packet tracer if it behaves like GNS3. In real world one side should give the clock as this is a synchronous serial line.

Hope to help

Giuseppe

[Searon]

Oh, now it works! I removed the previous login, but it seems you need to configure both username and passwords on both routers! The moment I added username brussel password cisco back to the router brussel the line went on again!

[Giuseppe Larosa]

Hello @Searon ,

you are right it is a mutual authentication using locally defined username/password pairs so both username must be defined on both routers.

Hope to help

Giuseppe

[paul driver]

Hello

You could also use unidirectional authtentication so then you would not require both usernames on each host.

brussel
username isp-1 password cisco

int s0/0/0
encapsulation ppp
clock rate 64000
ppp authentication chap callin < one way authentication
ppp chap hostname brussel

isp-1
username brussel password cisco
int s0/0/0
encapsulation ppp
ppp chap hostname  isp-1

or two way authentication both rtrs.
int x/x
no ppp chap hostname xxx
ppp authentication chap callin