OSPF Areas

The Impact of a Link-state Change

• OSPF is link-state
  • Is it up/down, what is the speed? these are things that OSPF routers will be discussing with each other.
  • Based on these paths what is the best one to take?
• Link State Advertisements are used
• LSA's are generated when a link changes state
• Neighbors perform recalculation
  • OSPF routers in the same subnet are known as neighbors
  • Form neighbors via hello messages sent on the multicast address 2224.0.0.5
  • Default Hello timer is 10 seconds
• Best routes are installed in the routing table

OSPF Storytime...

• It can lead to high utilization
• A big company with:
  • warehouses
  • Datacenter with fast switches
  • Headquarters were they have many employees working

Advantages of Multi-area OSPF

• Summarization
  • Dijkstra takes more CPU, therefore more time.
• Isolation of topology changes
  • An area can have multiple subnets
• Reduces complexity
  • Clean routing tables to look at
• Improved stability
• Reduced hardware requirements
  • It is really nice to go into existing hardware, tweak and tun it and get even more out of it.
  • Why don't we put one general route out there
    • It isolates topology changes
    • It will go from a large database that can be impacted by topology changes to a smaller database (calculating less often).
• Redistribution
  • Perform tuning for network environments

OSPF Router Types

• Internal Router
• Area Border Router (ABR)
• Autonomous System Boundary (ASBR)
• Backbone
  • Internal router with area 0
• Sometime we have mergence
  • We can connect a RIP environment with an OSPF environment.

OSPF Area Operations

• ASBR