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Split OSPF Area 0 Behavior – What happens?

Last updated on December 27, 2018

        To make my question as simple as possible, I put together a quick scenario:
  1. Area 0 spans two data centers with multiple routers/networks in each.

  2. Area 0 has two ABR's connected to area 1 that connect to the same router in area 1. One area 0,1 ABR is in each data center. These ABR's also happen to be WAN routers, so they are used to interconnect the data centers as well in area 0. Area 1 is a standard area.

  3. Area 0 becomes partitioned (split) due to WAN/MAN link(s) between the data centers going down.

See diagram: Area 0 Split Based on reading the OSPFv2 RFC and a lot of other info on the Internet, I assume the following: A. A partitioned backbone acts as two separate area 0's and they no longer know about each other. B. Each area 0 is fully functional for intra-area routing within its own partition. C. Functionality between each area 0 and area 1 is fully functional. Area 1 gets area 0 summaries from each partition and the ABR's send area 1 summaries into each area 0 partition. External routes from each area 0 partition also flood into area 1, along with type 4 LSA's for ASBR's. D. Routers in area 0 A can't get to networks in area 0 B, and vice versa because there is no virtual link through area 1. Since there is no virtual link, no routing information from area 0 A will be shared with area 0 B that an ABR learns in area 1 (and vice versa). Am I correct, or am I misunderstanding something? My assumptions are based on the distance vector / split-horizon behavior between areas. In the quick drawing I did, that means that R1 will flood area 0 summaries into area 1, which R4 will learn in area 1, but R4 will assume the area 0 it is directly connected to is the only area 0, so it won't even think about sending any routing info from those summaries "back into area 0". Obviously this isn't desired behavior for a working design, but if I'm understanding correctly, then this is how I want my network to fail if my two data centers lose connectivity between each other. I've inherited a single area OSPF domain and I want to break all branch site routers off into a separate area so that if area 0 splits, traffic between data centers won't traverse branch site WAN links through routers that are connected to both data centers. I also do some OSPF to BGP redistribution and I don't want an ASBR at one data center redistributing routes from networks at the other data center if its only path to them requires it to transit a dual homed branch office on a slow link. Thanks for any insight.

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