[nsp] Dial-Backup for PPPoE-Dialer?
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
dsl operating-mode annexb-ur2
interface atm0.1 point-to-point
pppoe-client dial-pool-number 1
ip address negotiated
ip mtu 1492
ip nat outside
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.
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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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