Split OSPF Area 0 Behavior - What happens?

Question

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:

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.

Answer 1

After more thoroughly combing through the RFC, I'm certain that the expected behavior I mentioned in my post is accurate.

Intra-area routing within each area (area 1, area 0(a), and area 0(b)) will work as expected.

Area 0 summaries will still flood to area 1 from each area 0 partition, giving area 1 all necessary routing information for both area 0 partitions.

Area 1 summaries will still flood to both area 0 partitions from area 1, allowing both area 0 partitions to know about area 1 routes.

Area 1 will not send area 0 summaries back into area 0, so split area 0 partitions will no longer know about each other and will not try to transit area 1 to get to each other (unless a virtual link through area 1 is created).

This is my desired behavior for a failure scenario based on my circumstances. If all of the redundant links between my data centers go down, or I need to perform certain types of maintenance, I don't want data center to data center traffic (mostly backups / off-site replication) saturating my branch site WAN links. I could apply policing on the WAN links for transit traffic between data centers, but the little bit of bandwidth I could give that traffic would be useless anyway - and that would require my team to maintain more config.

If anyone has any questions about this, feel free to comment and I will try to find the answers.

Answer 2

I think you're correct. The key point is ABR (R1 and R4), since they decide the flooding between areas and exchange area information in backbone area. However, the idea can be easier to understand. In RFC2328:

3.1. The backbone of the Autonomous System

The OSPF backbone is the special OSPF Area 0 (often written as Area 0.0.0.0, since OSPF Area ID’s are typically formatted as IP addresses). The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established/maintained through the configuration of virtual links.

3.7. Partitions of areas

However, in order to maintain full routing after the partition, an address range must not be split across multiple components of the area partition. Also, the backbone itself must not partition. If it does, parts of the Autonomous System will become unreachable. Backbone partitions can be repaired by configuring virtual links (see Section 15).

So we know an OSPF autonomous system can only have a backbone area. Based on the scenario you presented, after backbone area is splited, it's no longer a single OSPF autonomous system, but two OSPF autonomous systems [Area 0(a), Area 1] and [Area 0(b), Area 1]. So both autonomous systems have routing information of Area 1, which is the behavior you desired.

Answer 3

In short ABR will not pass type 3 LSAs to any other area unless it is received over the area 0. So here in this case, R1 or R4 are receiving the split area 0's type 3 LSAs, and they don't pass it back to area0s or any other area. This is the original loop avoidance of OSPF.

Thanks, Madhu