ISIS Routing – Maximum Number of ISIS Routers in an Area or Level

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.

1. R5 sends a Type1 LSA containing the /32 Loopback
2. R1 (Area 5 ABR), is connected to Area 0; it translates the Type1 LSA into a Type3 LSA with a cost of 3.
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.

¹_{RFC 3509 describes Cisco IOS's ABR behavior}

You can have multiple ABRs which connect an area to Area 0. With a stub area, a router in the area could choose the best ABR in the path to the destination, but with a totally stubby area, it will just choose the nearest ABR.

For instance, if you have two ABRs for your area (Area 1), each ABR will have, at least Area 0 and Area 1 interfaces, but one of them could also have an Area 2 interface, or it could be directly connected to an Area 2 ABR with a low cost, but the other ABR would have a higher cost to get to Area 2. It would make sense, when sending traffic to Area 2, to send it to the ABR with the lowest cost to Area 2, but a totally stubby area doesn't know how to do that.