Before today I undestood that between Areas we lose the detailed OSPF database that fully describes all the links and routers in the Area allowing each router to fully construct a topology and compute shortest paths. Between areas we rely on the summary information we get from the ABR's about other parts of the network and this limited view is similar to distance vector behaviour. The Area 0 concept enforces split horizon type behaviuor to ensure this summary information does not create loops between the Areas of OSPF. (I have seen Petr's blog about Loop prevention in OSPF and how you can get summary ifnormation to pass without going through 0 which in an excellent piece of work as always)
Today I learnt that an OSPF border router will not create a summary route in the connected area if it can't install the OSPF route in the RIB, so no database entry and therefore not passed on to other routers in the connected Area (RIP, EIGRP anyone? ;) ). Wow, it is more Distance Vector than I thought! This may be old news to a lot of seasoned vets but I thought it was worth sharing. So no summary route, no database entry, nothing.
I was using the INE topology in GNS 3 and running OSPF Area 0 covering the Frame and P2P link up to R4. R5 was configured with Vlan 58 as Area 3 and announcing this into Area 0 so I was expecting R1, R3 and R4 to annouce this into their connected Areas. I had BGP running on R5, R4 and R6 (seperate AS so eBGP AD 20) and had announced Vlan 58 into BGP. This stopped R4 from announcing it in Area 1. Wow.
Nick