OSPF (Open Shortest Path First) is a link-state routing protocol that allows routers to dynamically exchange routing information with other routers within an autonomous system (AS). In OSPF, all routers within the same area must share the same topology information, and are considered part of the same OSPF domain.
In OSPF, there is no specific limit on the number of routers that can be supported in a single area, although it is recommended to keep the number of routers in an area to a reasonable limit for optimal performance. The actual number of routers that can be supported in an area will depend on various factors such as the network topology, the amount of traffic being routed, and the resources available on the routers.
* OSPF Terminology
- Neighbor - connected (adjacent) router that is running OSPF with the adjacent interface assigned to the same area. Neighbors are found by Hello packets (unless manually configured).
- Adjacency - logical connection between a router and its corresponding DR and BDR. No routing information is exchanged unless adjacencies are formed.
- Link - link refers to a network or router interface assigned to any given network.
- Interface - physical interface on the router. The interface is considered a link when it is added to OSPF. Used to build link database.
- LSA - Link State Advertisement, data packet contains link-state and routing information, that is shared among OSPF Neighbors.
- DR - Designated Router, chosen router to minimize the number of adjacencies formed. The option is used in broadcast networks.
- BDR -Backup Designated Router, hot standby for the DR. BDR receives all routing updates from adjacent routers, but it does not flood LSA updates.
- Area - areas are used to establish a hierarchical network.
- ABR - Area Border Router, router connected to multiple areas. ABRs are responsible for summarization and update suppression between connected areas.
- ASBR - Autonomous System Boundary Router, router connected to an external network (in a different AS). If you import other protocol routes into OSPF from the router it is now considered ASBR.
- NBMA - Non-broadcast multi-access, networks allow multi-access but have no broadcast capability. Additional OSPF neighbor configuration is required for those networks.
- Broadcast - Network that allows broadcasting, for example, Ethernet.
- Point-to-point - Network type eliminates the need for DRs and BDRs
- Router-ID - IP address used to identify the OSPF router. If the OSPF Router-ID is not configured manually, a router uses one of the IP addresses assigned to the router as its Router-ID.
- Link State - The term link-state refers to the status of a link between two routers. It defines the relationship between a router's interface and its neighboring routers.
- Cost - Link-state protocols assign a value to each link called cost. the cost value depends on the speed of the media. A cost is associated with the outside of each router interface. This is referred to as interface output cost.
- Autonomous System - An autonomous system is a group of routers that use a common routing protocol to exchange routing information.