Once an interviewer asked me, "Can we use OSPF without area 0?" I was not able to answer at that time.
Later on, I searched, but I did not not find a satisfactory answer for this question. So, can we really use OSPF without Area 0?
Routing – OSPF Without Area 0
ospfrouting
Related Solutions
What does the topology look like? How are you doing routing today towards these networks? Or is the router being deployed now? Do you have an OSPF capable router connected to this router that is not running OSPF?
If you do then I suggest you redistribute static on this router. Something like:
ip prefix-list static-routes permit 10.0.0.0/24
ip prefix-list static-routes permit 10.0.10.0/24
ip prefix-list static-routes permit 10.0.20.0/24
route-map static-allowed permit 10
match ip address prefix-list static-routes
route-map static-allowed deny 20
The explicit deny at the end is not really needed as there is an implicit deny but it makes the logic a bit more clear.
Then under the OSPF process:
router ospf x
redistribute static subnets route-map static-allowed
OSPF Backbone
Why is area 0 the backbone area in OSPF? Why must all other areas connect to it?
This is explained very well in RFC 3509, Section 1.21:
1.2 Motivation
In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone. The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem. OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database.
OSPF is usually considered a link-state protocol. What some people miss is that OSPF uses both link-state protocol and distance-vector protocol algorithms.
- Routes within the backbone, or a non-backbone area are computed as a link-state protocol does (ref Dijkstra's algorithm).
- When OSPF must carry non-backbone routes through the backbone, it uses some distance-vector behavior (i.e. parts of the Bellman Ford algorithm) to propagate Type3 LSA metrics into non-backbone areas.
Simple example of OSPF's distance-vector behavior:
<-- Area 5 --><-- Area 0 --><-- Area 4 -->
R5-----------R1-----------R2------------R3---------------------R4
Cost 3 Cost 5 Cost 7 Cost 12
LSA--> LSA-->
Type3 LSA Type3 LSA
{From R1} {From R2}
R5 cost is 3 R5 cost is 8
Consider what happens to a /32 Loopback route for R5.
- R5 sends a Type1 LSA containing the /32 Loopback
- R1 (Area 5 ABR), is connected to Area 0; it translates the Type1 LSA into a Type3 LSA with a cost of 3.
- R2 (Area 4 ABR) receives R1's Type3 LSA (metric 3) and changes the metric to R5's Loopback, based on R2's cost to R1. Now R2's Type3 LSA for R5 has a cost of 8. This is the distance-vector behavior I mentioned above.
Requiring all non-backbone routes to go through the backbone is a loop-prevention mechanism.
Connecting non-backbone OSPF areas at an ABR
If 2 areas aren't connected through area 0 (discontiguous), how does OSPF behaving as a link state protocol increase the possibility of routing loops?
As we saw above, OSPF uses distance-vector behavior to send routes through the Area 0 backbone. Distance-vector protocols have well-known limits, such as the count-to-infinity problem. OSPF would be vulnerable to the same issues, if we didn't have boundaries on its behavior.
1RFC 3509 describes Cisco IOS's ABR behavior
Best Answer
There is only one way to use OSPF without an Area 0, and that is to use OSPF with a single area. If you only have one OSPF area, you can number it any way you like, but if you have even two areas, you must have an Area 0.