Open Shortest Path First (OSPF)

OSPF Introduction

• Open standard
• Cisco proprietary - vs. 3rd part
  • Uses Dijkstra algorithm.
• Routers, switches, firewalls, load balancers, WLCs, etc
• Windows, Linux, BSD, OSX

Open Shortest Path First

• Link-state
  • Build a map
  • Calculate the path
    • Using Dijkstra algorithm
    • When there is changes on the path, things constantly going up and down.
    • Everyone recalculates best path again
    • Everytime this happnes the router has to do a lot of work to reestablish a path for everyone.
• Scalable
  • Multi-area
    • OSPF is big enough to scale to areas
    • When there is a problem we can isolate it to a smaller group
  • Tunable

Steps of OSPF

• Enabled per interface
• Neighbor relationships
• Link-state database
• Dijkstra algorithm
  • Responsible for calculating the shortest path first
  • Also used by the mapping system (GPS, google maps, etc.)
• Optimal paths
  • Every single router running OSPF considers itself as the center of the universe
  • Optimal paths are pushed to Routing Information Base (RIB)
  • Administrative distance
    • Every router information has a different value to it.
  • Still competing with things like Static route, direct, etc...

OSPF Sample Configuration

• 192.168.45.0/24

• The middle one is EXACT
  • Only want to do OSPF in the interface with this exact IP
• The bottom one is running EVERYWHERE
  • Include any interface that I have
  • Kind of like the autopilot
  • The downside is that it will include everything that you have, but maybe you want to keep some things out of OSPF