OSPF - Design Principles

• Number of neighboring routers - no more than 60
• Number of routers in an area - no more than 50
• Number of areas connected to a router - no more than 3
• The router acting as the DR should be the highest spec.

OSPF Area Design

• The amount of data in an area impacts OSPF performance
• Use STUB / TOTAL STUB areas where possible
• Areas should be designed around geography and functional boundaries
• Summarization minimizes route changes (Flapping) impact
• Large networks have only ABRs in area 0
• Contiguous, split-able addressing

Typical OSPF Hub and Spoke Design

• Use STUB areas whenever possible
• Use NSSA to handle expansion oddities
• Area 0 ABRs are summary points
• Keep area 0 small and very stable

Improving OSPF convergence time

All protocols must detect, propagate, process, and update

Cisco implements OSPF Exponential backoff algorithm for LSA generation and SPF

LSA generation:

• LSA-START: Initial delay to generate LSA (0ms)
• LSA-HOLD: minimum time before flooding (5s, Increases Exponential)
• LSA-MAX-WAIT: maximum time to wait before flooding (5s)

SPF Algorithm:

• SPF-START: time to wait before initially running SPF
• SPF-HOLD: time between consecutive SPF runs
• SPF-MAX-WAIT: maximum time to wait between SPF runs