OSPF Path Selection

OSPF Path Selection

1. OSPF Cost Calculation ( RFC 2328 - section 16)
1. Route LSAs include cost attribute for each link ( value 0 to 65535)
2.  Lowest end-to-end cost
3. Uses bandwidth based cost but per RFC ( arbitrary) 
• OSPF uses bandwidth based cost
• cost = Reference_BW/Interface_BW
• Cost can be changed by
• interface bandwidth ( under interface mode)
• interface ip ospf cost ( under interface mode)
• Process auto-cost ( under router ospf mode)
• Process neighbor x.x.x.x cost xx ( under router ospf mode)
4. Virtual links inherit their cost from SPT cost between the virtual link endpoints
1. may exceed maximum link cost
2. OSPF Path Selection Order
1. Intra Area Routes (O)
2. Inter Area Routes (OIA)
3. External Type 1 ( E1)
4. NSSA Type 1 ( N1)
5. External Type 2 (E2)
6. NSSA Type 2 ( N2) 
3. Traffic Engineering OSPF Path Selection

Reference
IETF-RFC 2328
INE-OSPF-Path Selection