Issue:
Can the P routers be configured to automatically bring up a back-up line if the PPPoE connection fails?

Solution:

The 1730 and 2600 routers can be configured to bring up a back-up line if the PPPoE connection fails.
The valid connection types for back-up are ISDN and leased line. 
There is only support for one PPPoE connection.

The 840 and 850 routers currently do not support a back-up link.