What is OSPF?

OSPF (Open Shortest Path First) is a link state networking protocol. Each router broadcasts a packet that describes it own local links. Routers collect information from these broadcast packets to build their own network routing tables. These packets that describe the local links are short and cause less traffic congestion than Routing Information Protocol (RIP), which broadcasts large routing tables.

OSPF bases its routing decision on the least-cost path. The cost is administered value, usually based on line speed. If there are multiple areas in an OSPF domain, there must be a backbone area, identified as area 0. When areas are configure in the OSPF domain, there are four basic router classifications:

• Internal Router -- router with all directly connected networks belonging to the same area. Routers with only interfaces in the backbone area also belong to this category and have a single link-state database.
• Area Border Router (ABR) -- router that has directly connected networks belonging to multiple areas. ABRs have multiple link-state databases, one for each area, including the backbone. ABRs summarize the networks in their areas and advertise them onto the backbone area. The backbone, in turn, distributes the information to the other areas.
• Backbone Router -- a router that has at lease one directly connected network in the backbone area. This includes all ABRs. However, not all backbone routers must be ABRs. A backbone router that has all its interfaces in the backbone would be an internal backbone router.networks in their areas and advertise them onto the backbone
• Autonomous System Backbone Router (ASBR) -- router that has directly connected interfaces in non-OSPF networks. These networks are then brought into an OSPF domain. An ASBR is independent of the other router classifications.