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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