OSPF Overview

OSPF is a link-state routing protocol developed by the IETF as a replacement for RIP. It uses Dijkstra’s Shortest Path First algorithm to calculate a loop-free graph describing the shortest path to every known destination.

It is a very hierarchical protocol. Here are some of the items we will review.

There are 4 router types in OSPF:

  • Area Router
  • Area Border Router
  • Backbone Router
  • AS Boundary Router

The network type was created in order to deal with the different characteristics of the different media types. Network types do not have to match between neighbors, they just have to be compatible. To be compatible, they have to share the support of Type-2 Network LSAs.

There are 6 Network Types in OSPFv2:

  • Broadcast
  • Non-Broadcast
  • Point-to-Point
  • Point-to-Multipoint
  • Point-to-Multipoint Non-Broadcast
  • Virtual link

The network type controls:

  • How are hellos and updates sent?
  • Who forms adjacency?
  • How is the next-hop calculated?

There are 6 types of routes in OSPF:

  • intra-area routes
  • inter-area routes
  • External Type 1 (Closes to Destination - based on external metric)
  • External Type 2 (Hot Potato - based on forwarding metric (closest ASBR))
  • NSSA Type 1
  • NSSA Type 2