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Emulate dialup links with serial lines

I had to figure out various PPP parameters (and associated Cisco IOS behavior) and didn't have real dial-up equipment in my lab setup. I could have gone with PPPoE, but it turned out it's way simpler to emulate dialup connections (at least the PPP negotiations work as expected) on fixed serial lines. This is the minimum setup you need on the “caller” side …

interface Serial1/0
 ip address negotiated
 encapsulation ppp
 ppp authentication pap optional
 ppp pap sent-username client password 0 client
… and this is the “server”-side configuration:
interface Serial1/0
 ip address
 encapsulation ppp
 peer default ip address
 ppp authentication pap callin
username client password client
To trigger PPP negotiations, shut down and re-enable the serial interface on either side.

Note: As I'm using PAP authentication, I could use the more secure username secret configuration command, which would not work with CHAP.


  1. Please check "debug ppp auth". Is "dial-in" option really have influence?

  2. Based on "debug ppp negotiation" it looks like both ends treat the connection as "dial-in". The only way to have one-way PAP is to configure "ppp authentication pap optional" on one end.

  3. I used the AUX port on the router and connect them together using a null modem cable. If you really want to simulate dialup you can do dialer profile and put the AUX port in a dialer pool and create a dialer interface and associate the dialer pool to it. You can then enable ppp and all related PPP configs.

  4. Using clear interface Serial1/0 is more convenient way to force ppp renegotiation than shutdown and no shutdown in sequence.

  5. Did you try to use "ppp direction " command ?

  6. IMHO, on "client" side you don't need "ppp authentication pap" command as it demands *peer* (your "server") to authenticate...


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Ivan Pepelnjak, CCIE#1354 Emeritus, is an independent network architect. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.