Understanding OSPF

OSPF is an Interior Gateway Protocol (IGP) designed to distribute routing information between routers in the same Autonomous System (AS).

The protocol is based on link-state technology with several advantages over distance-vector protocols such as RIP:

No hop count limitations exist.
Multicast addressing sends routing information updates.
Updates are sent only when network topology changes occur.
Networks are logically defined by dividing routers into areas.
External routes injected into the AS are transferred and tagged.

However, OSPF has some disadvantages:

OSPF is CPU and memory intensive because the SPF algorithm must maintain multiple copies of routing information.
It is more complex to implement than RIP.

Terminology

Review the following terms to understand how OSPF operates. You will encounter these terms throughout the documentation.

Neighbor: A connected (adjacent) router running OSPF with the adjacent interface assigned to the same area. Hello packets discover neighbors unless you configure neighbors manually.
Adjacency: A logical connection between a router and its corresponding DR and BDR. Routers do not exchange routing information unless adjacencies are formed.
Link: A network or router interface assigned to any given network.
Interface: A physical interface on the router. When you add the interface to OSPF, it becomes a link used to build the link database.
LSA: Link State Advertisement, a data packet with link-state and routing information shared among OSPF neighbors.
DR: Designated Router, a router chosen to minimize the number of adjacencies formed. OSPF uses this option in broadcast networks.
BDR: Backup Designated Router, a hot standby for the DR. The BDR receives all routing updates from adjacent routers but does not flood LSA updates.
Area: A logical grouping to establish a hierarchical network.
ABR: Area Border Router, a router connected to multiple areas. ABRs are responsible for summarization and update suppression between connected areas.
ASBR: Autonomous System Boundary Router, a router connected to an external network (in a different AS). A router becomes an ASBR when you import routes from other protocols into OSPF on that router.
NBMA: Non-broadcast multi-access networks. These networks allow multi-access but have no broadcast capability. You must configure OSPF neighbors manually on these networks.
Broadcast: A network with broadcast capability. For example, Ethernet.
Point-to-point: A network type where DRs and BDRs are not required.
Router-ID: An IP address used to identify the OSPF router. If you do not configure the OSPF router-id manually, the router uses one of its assigned IP addresses as its Router-ID.
Link State: The status of a link between two routers. It defines the relationship between a router's interface and its neighboring routers.
Cost: A value assigned to each link by link-state protocols. The cost value depends on the speed of the media. Each router interface has an associated cost on its output side, referred to as the interface output cost.
Autonomous System: A group of routers that use a common routing protocol to exchange routing information.

Supported standards

RFC 2328 - OSPF Version 2
RFC 3101 - The OSPF Not-So-Stubby Area (NSSA) Option
RFC 3630 - Traffic Engineering (TE) Extensions to OSPF Version 2
RFC 4577 - OSPF as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)
RFC 5329 - Traffic Engineering Extensions to OSPF Version 3
RFC 5340 - OSPF for IPv6
RFC 5643 - Management Information Base for OSPFv3
RFC 6549 - OSPFv2 Multi-Instance Extensions
RFC 6565 - OSPFv3 as a Provider Edge to Customer Edge (PE-CE) Routing Protocol
RFC 6845 - OSPF Hybrid Broadcast and Point-to-Multipoint Interface Type
RFC 7471 - OSPF Traffic Engineering (TE) Metric Extensions