This question regards a very specific corner of the OSPF PE-CE implementation in Hub & Spole topology on L3 MPLS VPN
Concerning the area of ospf on each and every pe-ce connection between the spokes and the hub CE's to the PE's.
In this scenarion a single vrf is configured per CE-PE and 2 vrf are configured for Hub-PE & Hub-CE connection
one vrf for import ,and one for export.
on all PE-CE links i have OSPF configured.
each connection is in a differnet OSPF area.
Alegedly, there should not be any problem - it shoudln't matter which ospf area is configured on the links - except in the case of Hub-PE to Hub-CE connection. - and here is why....
from now on i'm refering only to the HUB-PE vrf's.(import &export)
The OSPF instance under the import vrf is redistributing routes from BGP to OSPF and advertises them to the HUB-CE.
in the case of L3 vpn there is a special behaviour that makes the HUB-PE ospf vrf instance become ABR area 0 in order to advertise routes learned from the BGP to OSPF as internal T3 routes instead of external T5.
The Hub-CE accepts those routes as summary LSAs and advertises them as T3 to his neighbors in the same area !!
***know that the OSPF area on both vrf is any area other then area 0 !!
The OSPF instance on the export vrf is in the same area as the import vrf OSPF.
So, routes are advertised back to the OSPF instance on the export vrf.
However - those T3 lsa are not selected in the route table of the export VRF -they exist in the database , but they are not used as routes. -therefore the export vrf dosen't perform it's destiny and no prefixes from the spokes are returned back to the network through the HUB-PE.
When im configuring the OSPF area as 0 on both links of HUB-PE to HUB-CE it works.
LSAs are exactly the same and distribution is the same - the only differnce is the activation of those routes on the export vrf.
I've attached the layout of the network for easier understanding , and i've attached a CCIE MPLS example of this scenarion
You can see that in the CCIE example - there is area 0 configured in the HUB-PE HUB-CE links and not a different area.
this scenario is not regarding DN bit/route-type/domain-Tag ... on every implementation i've configured the same, and disabled the loop checking mechanism of OSPF(regarding the DN,route0type,domain-tag..and so on)
I haven't found any documentation regarding Hub & Spoke scenarios with attention to OSPF areas on the HUB links.
I've read RFC4577, and rosen-draft (ppvpn,ospf-bgp-mpls)but i haven't found any relation to that specific case.
In my opinion it is caused from wrong implementation of OSPF as PE-CE protocol on the vrf - and it requires OSPF area 0 connected to it.
LINK TO CCIE MPLS studycase : http://fengnet.com/book/IOS_MPLS/ch14lev1sec7.html