Why backbone area in ospf called Area 0?Could any router in ospf backbone area be configured as a stub?If 'YES' any disadvantages by doing that?If 'NO' why?
And why all the other areas must be connected to the backbone area?
Area0 is a reserved area. It is always the backbone area.
A stub connected to the backbone should be its own area, as it will only get a default route from the ABR. If you need more than a default, use nssa, which allows Type 7 LSAs that are converted to Type 5 LSAs at the ABR.
I do have one complaint with that doc though. It says Area0 is required, but this is really dependant on your OSPF design.
I am currently implementing OSPF for 5 Motorola Canopy Access Point sites. All five sites are part of a single area, area 1. Should we need it, I can easily add Area0 at the core, but it is not required in a single area environment.
All areas need to be connected to the backbone area as the backbone needs to know about all areas in its domain, however this does not need to be a physical connection. You can use virtual links for areas that do not have a physical connection to the backbone.