Using OSPF areas solves many of the most common problems with running OSPF in larger networks. OSPF areas break up the network so that routers in one area know less topology information about subnets in the other area and they do not know about the routers in the other area at all. Smaller-topology databases cause routers to use less memory and take less processing time to run SPF.
An Area Border Router (ABR) is on the border between two different areas.
OSPF Design Terminology
|Area Border Router (ABR)||An OSPF router with interfaces connected to the backbone area and to at least one other area.|
|Autonomous System Border Router (ASBR)||An OSPF router that connects to routers that do not use OSPF for the purpose of exchanging external routes into and out of the OSPF domain.|
|Backbone router||A router in one area, the backbone area.|
|Internal router||A router in a single nonbackbone area.|
|Area||A set of routers and links that share the same detailed LSDB information, but not with routers in other areas, for better efficiency.|
|Backbone area||A special OSPF area to which all other areas must connect. Area 0.|
|External route||A route learned from outside the OSPF domain and then advertised into the OSPF domain.|
|Intra-area route||A route to a subnet inside the same area as the router.|
|Interarea route||A route to a subnet in an area of which the router is not a part.|
|Autonomous system||In OSPF, a reference to a set of routers that use OSPF.|
It is important to note the difference between the summarized information in OSPF and route summarization. In this case, the term “summary” just means that a router inside one area receives briefer information in the LSA for a subnet, thereby decreasing the amount of memory needed to store the information. This process reduces the size and complexity of the SPF algorithm. In addition, the term “summary” can refer to a summary route configured in OSPF. OSPF manual route summarization reduces the number of subnets, which in turn also reduces the size and effort of the SPF calculation.
The dividing line between areas is not a link, but a router. OSPF uses the term Area Border Router (ABR) to describe a router that sits in both areas. An ABR has the topology database for both areas and runs SPF when links change status in either area. Although using areas helps scale OSPF by reducing the size of the LSDB and the time to compute a routing table, the amount of RAM and CPU consumed on ABRs can actually increase. As a result, the routers acting as ABRs should be faster routers with relatively more memory.
OSPF Area Design Advantages
Using areas improves OSPF operations in many ways, particularly in larger internetworks:
* The smaller per-area LSDB requires less memory.
* The router requires fewer CPU cycles to process the smaller per-area LSDB with the SPF algorithm, reducing CPU overhead and improving convergence time.
* The SPF algorithm has to be run on internal routers only when an LSA inside the area changes, so routers have to run SPF less often.
* Less information must be advertised between areas, reducing the bandwidth required to send LSAs.
* Manual summarization can only be configured on ABRs and ASBRs, so areas allow for smaller IP routing tables by allowing for the configuration of manual route summarization.