OSPF Areas are used for OSPF Scalability. In this post, we will have a look at many different topics about OSPF Area Types, their usage, limitations, different router types, and so on. In OSPF, we have in general two different area types.
OSPF Backbone Area and OSPF Non-Backbone Area. There are many different OSPF Non-Backbone Area types and we will cover each one of them, their use case, limitations, and benefits in this post.
OSPF Area 0 - OSPF Backbone Area
First of all, Let's start with Area 0. It is known as the Backbone area in OSPF and if there are many different areas, non-backbone areas can communicate with each other through OSPF Area 0. Let's use the below topology for the rest of the OSPF Area Types discussion.
Figure - OSPF Area Types
In the above topology, OSPF Area 0, which is OSPF Backbone Area, physically connects different OSPF Non-Backbone Areas. OSPF Router Types in OSPF Area 0 are known as Internal Backbone Routers. We don't have OSPF LSA restrictions when it comes to OSPF Area 0, all the LSAs are allowed in this OSPF Area, except Type 4 and Type 7. Type 4 LSA is used for AASBR reachability in Non-Backbone areas in Multi Area OSPF which will be explained in the post later. Before we continue OSPF non-backbone Area Types, let's briefly mention different OSPF Router types.
OSPF Router which connects different types of Areas known as OSPF ABR, OSPF Area Border Router. Based on the Standard OSPF RFC, if a router has an interface in two different OSPF Areas, it is considered an OSPF ABR. Two different OSPF Areas can be even two different OSPF non-backbone Areas. But implementation on vendor routers requires a router to have at least one interface in OSPF Area 0, in order to be considered as an OSPF ABR. OSPF ABR allows summarization between the Areas. Meaning, that if there are many /24 subnets, for example, all of them can be summarized as /18, /16, etc., and sent as less prefix to the other OSPF Area.
Sending summary route is good for OSPF Fast convergence, OSPF Troubleshooting, and overall OSPF High Availability. Also, OSPF ABR doesn't allow topology information to be shared between the OSPF Areas.Meaning, that OSPF Cost information of one OSPF Area is not known by other internal OSPF Area routers, because ABR in OSPF doesn't allow this information to be shared. It is also important for the scalability of OSPF networks.
In the above topology, R3 and R4 are the OSPF ABR Routers, connecting Area 1 and Area 2 to the Backbone OSPF Area respectively. Area id in OSPF is important in OSPF Design as we will see later in the post, we can create OSPF Hierarchical design by assigning OSPF Area IDs/numbers carefully.
OSPF ASBR, Autonomous System Boundary Router, is not a special OSPF Router Type actually. ASBR is a generic term that is used in many different architecture and frameworks in Networking. We use this term whenever one routing protocol advertises information in another routing protocol.
OSPF redistributes to EIGRP or vice versa, BGP redistributes to IS-IS or vice versa, and any routing protocol redistribution basically is done on the ASBR Router. Basically, a router which is connecting different Administrative domains is known as ASBR.When it is used in OSPF, it is called OSPF ASBR. In the above topology, R2 is the OSPF ASBR, that connects different routing domains to the OSPF network.
OSPF Stub Area
This is one of the OSPF Non-Backbone Area types. Stub Area doesn't allow OSPF Type 5 LSA. When there is no Type 5 LSA, no Type 4 LSA is generated by the OSPF ABR into the OSPF Stub Area.
The overall reason why we use OSPF Stub Area is, that we don't want to allow redistributed prefixes in Stub Area OSPF. But still, Type 3 LSA is allowed in OSPF Stub Area.
OSPF Totally Stubby Area
Another Non-Backbone area in OSPF is OSPF Totally Stub Area. In the Totally Stub area, in addition to Type 5 and Type 4 LSA, Type 3 LSAs are not allowed either. So, in addition to the redistributed prefixes, Inter-Area OSPF prefixes are not allowed as well.
Why we don't allow Inter-Area prefixes in Totally Stub Area in OSPF?
The reason is again, scalability. Whenever you restrict more LSA, it is good for overall network scalability. It might create sub-optimal routing but Network design is all about trade-offs. We lose something, we gain another. Let's have a look at the next, Single Area and Multi-Area OSPF.
Single Area OSPF
We can place all our Routers in a Single Area OSPF. It is also commonly referred to as Flat OSPF Network Design. All routers in OSPF Area 0 or any other OSPF Area. This means, that if you place all your routers in OSPF Area 100, it is still considered a Single Area OSPF. Because, each and every router inside that OSPF Area will share the same OSPF database, same reachability information, basically the same everything about each other!
OSPF Single Area design/Flat Design may not be scalable after some amount of OSPF Routers, and prefixes. Thus, for scalability, we usually consider hierarchy. A common way to deploy hierarchy in OSPF is explained next.
Multi Area OSPF
OSPF Multi-Area design involves OSPF Area 0, Bachone OSPF Area, as well as Non-Backbone Areas. In this design, topology information is not exchanged. Cost between the routers in OSPF Area 0 is only known by Internal OSPF Area 0 routers. Not by any other type of OSPF Area in the network.
Also, usually, Network Admins summarize the prefixes between the OSPF Areas, so less amount of information is exchanged between the areas. Or, they deploy the Non-backbone OSPF Areas as Totally Stub Area or NSSA areas so, they just send the default route into those OSPF Areas. Multi-Area OSPF design is covered here in more detail.
OSPF NSSA Area
OSPF NSSA Area is also a Stub area but it is called an OSPF Not So Stubby Area. You will understand the reason below. When we don't want to allow Type 5 LSA, which means redistribution from the Backbone Area but still allow the redistribution into the Non-Backbone OSPF Arwa, then we deploy OSPF NSSA Area. NSSA OSPF allows redistribution from the different domains but redistributed prefixes can't come from the Network Backbone/OSPF Area 0.
A router that connects OSPF NSSA Area and the OSPF Area 0 is known as OSPF NSSA ABR. Redistributed prefixes in NSSA are marked as Type 7 LSA. OSPF NSSA ABR translates OSPF Type 7 LSA to OSPF Type 5 LSA. If there is more than one OSPF NSSA ABR, the Router with the lower OSPF Router ID translates from Type 7 to Type 5.