[nsp] Dial-Backup for PPPoE-Dialer?

Gert Doering gert at greenie.muc.de
Thu Feb 12 08:49:42 EST 2004


Cisco's concept of "dialer interfaces are always up" has its merits, but
makes certain situations quite challenging.

Imagine the following scenario:

Cisco 836, at customer premises
  primary internet access: ADSL interface, with PPPoE (over ATM over ADSL)
  secondary access, for backup purposes: dialup via ISDN

the "standard" configuration for PPPoE access makes use of a dialer

vpdn enable

vpdn-group pppoe
  protocol pppoe

interface ATM0
    dsl operating-mode annexb-ur2
interface atm0.1 point-to-point
    description DSL-Pseudo-Dialup
    no shut
    pvc 1/32
      pppoe-client dial-pool-number 1
interface Dialer1
    description DSL-Pseudo-Dialup
    ip address negotiated  
    ip mtu 1492
    ip nat outside
    encapsulation ppp 
    dialer pool 1
    ppp authentication chap callin
    ppp chap hostname XXX
    ppp chap password YYY
ip route dialer1 20

this works fine.  Now, what to do if the ADSL line breaks?  The "dialer1"
interface will *always* be "up", due to the way dialer interfaces work.

One workaround we have found is to use "backup interface" from the
ATM0 interface to a dialer2 interface (ATM0 goes down, dialer2 changes
from "standby" to "up" and a floating static route with a better distance
than the route to dialer1 goes active).

This works well for physical line outages.  

It doesn't work for a number of other things that tend to go wrong in
ATM/DSL/PPPoE scenarios with a wholesale/incumbent Telco in between:

 - physical line up, but ATM circuit screwed
 - physical line up, ATM fine, but PPPoE fails to log in due to
   NAS / radius server breakage at the Telco

in all these cases we want to have the router switch over to ISDN dialup,
but I just can't find any trick how to do that.

Other routers (Bintec comes to mind) disable the PPPoE interface for 
a configurable amount of time after a PPP negotiation failure, and 
subsequently fall back to the PPPoISDN interface.  Which is exactly 
what we'd need here.


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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de

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