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OSPF Protocol Basic Overview

OSPF Protocol Basic Overview

What is OSPF


Language-wise it stands for Open Shortest Path First, and Family wise it belongs to the Link-State Interior Gateway Dynamic Routing Protocols. Done with the CV yet?, OSPF is an open standard internal routing protocol that is supported across all the different vendors manufacturing networking platforms. In this article, we will review the basics and specs of this protocol, and see its own unique features.


OSPF Neighbor States

As a start, the OSPF routing protocol uses a multicast hello message that is destined to the OSPF Multicast address of 244.0.0.5 seeking any possible other OSPF routers in the area. This message keeps repeating every 10 seconds by default, and that will be out of the interfaces that announced an OSPF configuration, which depends on how you configured it + the vendor-specific configuration template. Upon receiving a multicast hello message from another router we already sent it a hello message earlier, and that should be within the dead timer of 40 seconds maximum (by default). An OSPF neighbor process will start by:

  • Init:

    • at the moment of confirmation that a bidirectional multicast hello has initiated

  • 2-Way:

    • communication from the 2 parts has successfully occurred

  • ExStart:

    • OSPF router-id gets exchanged and database information begins to be shared

  • Exchange:

    • link states bidirectional communication and exchange

  • Loading:

    • final synchronization of LSA's

  • Full:

    • settlement and official neighboring announcement


OSPF Routing Protocol

Before starting routing, OSPF understands its neighboring environment by drawing a shortest-path tree SPT to those neighbors that it established a neighboring state with them earlier, based on the Dijkstra's Algorithm. Having the tree will allow the OSPF router to know how to reach each neighbor in the best way, and that will depend on the OSPF metric calculated per neighbor. named as the "Cost".


OSPF at Rest

With no triggers or events, OSPF routers will be calm, with no need to exchange any routes or LSA's, leaving the databases named LSDB's as steady, identical between routers, and containing the same types and amounts of LSA's. Only the OSPF hello message will be forever bidirectionally exchanged to maintain the neighbor state.

Author:

Orhan Ergun
Orhan Ergun CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

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