[c-nsp] "Backup load" and "backup interface" command

[Kanagaraj Krishna]

Hi,
   I have a requirement from a customer asking whether its possible to use 
ISDN BRI as a backup (secondary) for his leased line (primary) in situations 
where the traffic load is high. The "backup load X X" command together 
with "backup interface" would be might be one of the solutions. My questions 
are:

- How effective would this solution be especially using BRI as the backup 
interface (most examples online uses leased line for secondary line)?
- How would the traffic (incoming&outgoing) be distributed when both the links 
are up during high traffic?
- Would it be a problem if the bandwidths are speed different between the 
leased line (example: 512K) and ISDN BRI (example: 128K)?

Apart to that I also have a questions on using "backup interface" for backup 
during primary line outages (without the load command).

- I've read that, using the "backup interface" command will only kick start 
the ISDN backup when the leased line is [physical down: line protocol down]. 
Is it true and if yes, what is the purpose because most of the situation would 
only have line protocol as down?
- Would using floating routes be a better solution in ISDN BRI backup 
scenarios?

Hope to be enlightened.

Thanks,
Kana