Understanding OSPF Terminology and Operation-1

By | October 15, 2013

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

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 Terminology

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

SPF Tree

OSPF Areas

  • 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

  • OSPF maintains three databases which are used to create three tables.DatabaseLSA
  • OSPF Areas

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.

2 layer herierical


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Understanding OSPF Terminology and Operation-2



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