[nsp] Dial-Backup for PPPoE-Dialer?

[Gert Doering]

Hi,

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
interface:

vpdn enable

vpdn-group pppoe
 request-dialin
  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 0.0.0.0 0.0.0.0 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.

Suggestions?

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