OSPF Area Types and Concept Explained

OSPF uses the area concept to provide scalability. An area is a group of contiguous networks. OSPF uses it to control when and how much routing information is shared across the network.

Traditional distance vector routing uses a flat network design. A flat network design causes scalability issues in large networks. It also reduces the overall network performance. Let us understand the limitations and issues the flat network design has.

In a flat network design, if a change occurs on one router, it affects every router in the entire network. If the network has only a few routers, the effect is negligible. However, if the network has many routers, the effect is enough to halt the entire network.

For example, a network has 1000 routers and serval thousand subnets. In this network, a link is flapping. Flapping links cause the connected routers to update their topology tables and inform connected routers about this change. Connected routers update their topology table and inform their connected routers. This process continues until all routers learn the change. This process happens each time when the link goes down or comes up. A flapping link triggers this process so rapidly that the routers fail to reach convergence.

A flapping link causing trigger updates is not only the problem the flat network design has. Apart from this, a flat network design also has the following issues.

  • In the flat network design, each router has to learn and save information about all routers and routes. Each router needs to maintain and process a large database. A larger database requires more memory and CPU cycles.
  • OSPF uses the SPF algorithm to compare and select the shortest path. It requires processing power that grows exponentially compared to the size of the topology database.
  • Managing and troubleshooting a flat network is also a tough task. If something goes wrong, administrators need to check the entire network.

To overcome the limitation of the flat network design, OSPF uses a hierarchical network design. It breaks the topology database into several smaller databases using OSPF areas.

With areas, it places each link into one area. Each router runs its SPF algorithm only on its area topology. It does not need to learn all the routes outside the area. It only needs to learn all routes that are inside the area. However, it needs to learn a summary of all routes available outside the area.

Let's take an example.

In the following network, Router-A only needs to learn the link-1, link-2, link-3, and link-4. It does not need to learn the link-10, link-11, link-12, and link-13. Router-AA will provide a summarized route for these links to Router-A. Similarly, Router-AA does not need to learn the link-1, link-2, link-3, and link-4. It only needs to learn the link-10, link-11, link-12, and link-13.

ospf area concept

Network size and OSPF areas

There is no rule regarding the network size, after which you should divide it into areas. Typically, networks larger than a few dozen routers benefit from areas. That is because the SPF process largely depends on the CPU processing speed, amount of RAM, and the topology size.

Let us take an example.

A network consists of 1000 routers and 2000 subnets. If the administrator divides the network into 100 areas, each area will cover approximately 10 routers and 20 subnets. With this setup, the SPF calculation on a single router only has to process a topology of 10 routers and 20 links instead of 1000 routers and 2000 links.

OSPF areas

OSPF supports a two-layer hierarchy: the backbone and areas connected to the backbone. It uses a 32-bit number to identify each area. The number can be written as a single decimal number (such as 1) or in a dotted-decimal format (such as 0.0.0.1).

The backbone area is compulsory. It represents the top-level hierarchy of the OSPF network. It uses the number 0. In other words, area 0 is the backbone area. Apart from area 0, all other areas are known as non-backbone or areas connected to the backbone. All other areas must have a link to the area 0. All areas inject routing information into the backbone. The backbone disseminates that information into other areas.

OSPF area route summarization

OSPF summarizes contiguous subnets. Through a correct IP addressing design, it can summarize routing information between areas. Route summarization reduces the amount of information that routers need to know. But it will work only when subnets are properly aligned.

The following figure shows an OSPF network. In this network, each area is assigned a contiguous Class B subnet. Through summarization of the border routers between areas, routers in other areas do not need to see all the Class B subnets. They only need to see the summarized network numbers for each respective area.

ospf terms

OSPF area terms

The following table describes the terms OSPF uses in the area concept.

Area Border Router (ABR) An area border router is a router that has interfaces in both backbone and non-backbone areas.
Backbone router A router having all interfaces in the backbone area.
Internal router A router having all interfaces in a non-backbone area.
Area A group of routers having the same routing information and topology database.
Backbone area Top-level area in the hierarchy. All other areas must connect to it.
Non-backbone area Apart from the backbone area, all other areas are non-backbone area
Intra-area route A route to a subnet inside the same area
Interarea route A route to a subnet outside the area

Backbone area

The backbone area is the central point of this implementation. Routers running in this area are required to maintain a complete database of the entire network. All areas need to connect with this area through a physical link or a virtual link.

Area off backbone

The area-off backbone is an extension of the backbone area. Routes running in this area maintain an area-specific database instead of the complete database. This feature reduces network traffic and speeds up the convergence process.

Roles of IR, ABR, and DR routers in an OSPF network

IR, ABR, and DR routers use LSA 1, LSA 2, and LSA 3 for many purposes. To understand these types in detail, we have to understand the role of IR, ABR, and DR routers in an OSPF network.

IR (Internal Router)

An IR router has the least responsibilities in the OSPF network. It maintains only area-specific routing information.

ABR (Area Border Router)

An ABR connects the area to another area. It keeps the area-specific routing information and summarized routing information of the entire network.

DR (Designated Router)

In an Ethernet/Broadcast environment, IR routers don't share routing information directly. They elect a DR router. The DR router is responsible for maintaining, sharing, and updating area-specific routing information.

Best practice for OSPF area design

To design the OSPF area, you should follow the following guidelines.

  • Always start with area 0
  • Plane IP addressing in such a way it produces contiguous subnets
  • Use a separate area for each subnet
  • Keep all routers belonging to the same subnet in the same area
  • Put all interfaces connected to the same subnet inside the same area
  • Configure at least one router in each non-backbone area as ABR

Advantages of OSPF areas

OSPF areas provide the following advantages.

  • It reduces the size of information each router needs to store and process.
  • Routers need less RAM to store the topology table.
  • Routers need fewer CPU cycles to run the SPF algorithm.
  • A flapping link in an area affects only the routers available inside the area. It does not affect the routers available outside the area.
  • It reduces the size of routing updates.
  • Routers consume less network bandwidth to share routing updates.

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