Summary:

• Link state
• Classless/VLSM
• IGP
• Dynamic
• Metric (cost)
• Trigger update
• Hop count unlimited
• Maximum path = 4
• AD = 110
• Multicast: 224.0.0.5 / 224.0.0.6
• Neighbor table
• Topology table (Contain all route and their attached link in the area or network)
• Routing Table (Contains the best path to the destination)
• Concept of Backbone Area & Non-backbone Area
• Concept of ABR, ASBR, DR, BDR

 

Introduction:

Open Shortest Path First (OSPF) is a routing protocol which was first defined as version 2 in RFC 2328. It is used to allow routers to dynamically learn routes from other routers and to advertise routes to other routers. Advertisements containing routes are referred to as Link State Advertisements (LSAs) in OSPF. OSPF router keeps track of the state of all the various network connections (links) between itself and a network it is trying to send data to. This makes it a link-state routing protocol. OSPF supports the use of classless IP address ranges and is very efficient. OSPF uses areas to organize a network into a hierarchal structure; it summarizes route information to reduce the number of advertised routes and thereby reduce network load and uses a designated router (elected via a process that is part of OSPF) to reduce the quantity and frequency of Link State Advertisements. OSPF does require the router have a more powerful processor and more memory than other routing protocols.

 

OSPF selects the best routes by finding the lowest cost paths to a destination. All router interfaces (links) are given a cost. The cost of a route is equal to the sum of all the costs configured on all the outbound links between the router and the destination network, plus the cost configured on the inteface that OSPF received the Link State Advertisement on.

This tutorial will focus on explaining the basic components of OSPF, the operation of OSPF, basic configuration of OSPF and finally close with troubleshooting techniques used to verify correct OSPF configuration and operation.

 

OSPF Router Types

In this tutorial, when speaking of an OSPF router, we are speaking of the OSPF routing process running on a given routing device. OSPF routers serve in various roles depending upon where they are located and which areas they participate in.

 

Internal Routers

An internal router connects only to one OSPF area. All of its interfaces connect to the area in which it is located and does not connect to any other area.

 

If a router connects to more than one area, it will be one of the following types of routers.

 

Backbone Routers

Backbone routers have one or more interfaces in Area 0 (the backbone area).

 

Area Border Router (ABR)

A router that connects more than one area is called an area border router or ABR. Usually an ABR is used to connect non-backbone areas to the backbone. If OSPF virtual links are used an ABR will also be used to connect the area using the virtual link to another non-backbone area.

 

Autonomous System Boundary Router (ASBR)

If the router connects the OSPF Autonomous System to another Autonomous System, it is called an Autonomous System Boundary Router (ASBR).

 

OSPF elects two or more routers to manage the Link State Advertisments:

 

Designated Router (DR)

Every OSPF area will have a designated router and a backup designated router. The Designated Router (DR) is the router to which all other routers within an area send their Link State Advertisements. The Designated Router will keep track of all link state updates and make sure the LSAs are flooded to the rest of the network using Reliable Multicast transport.

 

Backup Designated Router (BDR)

The election process which determines the Designated Router will also elect a Backup Designated Router (BDR). The BDR takes over from the DR when the DR fails.