Shortest Path First (OSPF)
OSPF is a standards-based link-state IP routing protocol described in RFC 2328.
- It was developed to meet RIP’s inability to scale beyond 15 routers.
- Proposed by IETF in 1988 and formalized in 1991.
- There are 2 versions; OSPFv2 is for IPv4 and OSPFv3 is for IPv6.
OSPF features include:
- Fast convergence
- Supports VLSM
- Efficient use of bandwidth -Routing changes trigger routing updates (no periodic updates)
- Supports large network size
- Routing based on best path selection
- Grouping of members into Areas
Link-State Protocol Characteristics
With link-state routing protocols, each router has the full picture of the network topology, and can independently make a decision based on an accurate picture of the network topology.
To do so, each link-state router keeps a record of:
- Its immediate neighbor routers.
- All the other routers in the network, or in its area of the network, and their attached networks.
- The best paths to each destination.
OSPF databases / tables:
- OSPF adjacency database =Neighbor table
- OSPF link-state database = Topology table
- OSPF forwarding database = Routing table
Link-state advertisements (LSAs)
Link-State Database (LSDB)
Shortest-Path First (SPF) Routing Algorithm
- Dijkstra algorithm
- Backbone (transit) and standard areas.
Types of OSPF routers:
- Internal router, backbone router, Area Border Router (ABR), Autonomous System Boundary Router (ASBR)
- Designated Router (DR) and Backup Designated Router (BDR)
OSPF Router Tables / Databases
To minimize processing and memory requirements, OSPF can divide the routing topology into a two-layer hierarchy called areas.Characteristics of OSPF areas include:
•Minimizes routing table entries.
•Localizes impact of a topology change within an area.
•Detailed LSA flooding stops at the area boundary.
•Requires a hierarchical network design.