Backbone Area 0 OSPF Why needed (in simple terms)

[nwekechampion]

Hi all,

Does anyone know why the ospf design dictates that we have all areas connected to the backbone and that areas that are dis-contiguous be connected via a virtual link?

When I tried configuring an area that wasn't connected to the backbone, I still had connectivity, only diference was that the routes got advertised in my RIB as LSA type3 (O IA) as opposed to intra-area (O), which isn't really a big deal to me as long as I have connectivity.

So my confusion is why do we need the backbone? is it strictly for adherence to the ospf design protocol?

Or am I missing something here?

Thanks

[mkazam001]

You can have multiple OSPF areas without an area zero, they just won't share routes.

Area zero, by design, is special in that other areas will accept and provide routes to it.  (area types and special OSPF statements define and control route exchanges between an non-zero area and area zero.)

Regards, mk

[Julio E. Moisa]

Hi

About....Does anyone know why the ospf design dictates that we have all areas connected to the backbone and that areas that are dis-contiguous be connected via a virtual link? You can use Virtual links or GRE Tunnels. 

Simple terms, the area 0 or backbone is used to interconnect all the other areas in order to have a better control or calculation of the SPF and avoid layer 3 loops. 

Hope it is useful

:-)

[nwekechampion]

HI Julio,

Does the virtual link created affect the lsa types between an Area that is not connected to the backbone that that which is.
In my lab, the backbone routes keep showing up as "O", instead of "IA". Whereas normally it should be just "IA" from the backbone in my routing table?

Thanks