OSPF | Open Shortest Path First | Quick Overview

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In this article, we will discuss about Open Shortest Path First (OSPF). It is an open standard routing protocol which is defined as RFC 2328 for IPv4 and RFC 5340 for IPv6.

ospf-routers-and-areas-summery

Quick Overview to OSPF

OSPF is one of the famous IGP protocols, which operate in a single autonomous system number. It uses Dijkstra’s algorithm, also known as SPF (Shortest Path First) algorithm in order to calculate the best path to the destination. Unlike other dynamic routing protocols, each router of OSPF knows the complete topology. However, the forwarding decision is taken on the basis of Best Route in the routing table. Like EIGRP, OSPF also works on the network layer of the OSI Model and its protocol number is 89.

OSPF Metric

OSPF uses path cost as a basis metric. This metric depends upon interface bandwidth. According to Cisco, the cost of 100Mbp/s link is 1, 10Mbp/s link is 10 and 1Mbp/s is 100 and so on. However, links which are greater than 100Mbp/s also have metric 1.

Different Types of OSPF Area

In order to divide large and complex OSPF network into small groups, OSPF introduces Area’s. In OSPF, the area is used for logical grouping of Hosts and Routers. With the help of these areas we can understand OSPF topology easily and router can make routing decisions easily. Cisco recommends, that we should put maximum 30 routers in an area.

Let’s continue!

Backbone Area

It is a Core Area of OSPF protocol. The backbone area is also known as Area 0 or Area 0.0.0.0. All other routers are connected to this area either directly or indirectly. Backbone area is responsible for forwarding routing information to all non-backbone area router’s. All OSPF areas must be connected to the backbone area.

Stub area

Stub area is the OSPF area which have following properties:

  • It is used to filter type 4 LSA and type 5 LSA.
  • After filtering type 4 LSA and type 5 LSA, it automatically generates a default route.
  • The default route is advertised by ABR in the form of LSA3.
  • We configure stub all the routers present in an area.
  • We can’t configure Area 0 as a stub area.
  • Stub area not allow a virtual-link.
  • However, Stub area doesn’t allow ASBR router, that’s why we can’t configure stub area near ASBR router.

When we configure an area as a stub, then it brake the neighborship with neighbor router and then send next Hello Packet with the stub-bit.

Let’s take a look, how we configure Stub Area!

GNS3Router(router-configure)#area 3 stub

Totally Stub Area

  • It filters LSA3, LSA4 & LSA5 or external as well as inter-area route.
  • It automatically generates Default route in the form of LSA3.
  • Needs to implement only on ABR router.
  • No virtual-link allowed.
  • We should declare all area router as stub routers.

This command can use to configure OSPF Totally Stub Area:

GNS3Router(router-configure)#area 3 stub no-summary

Not-So-Stub Area (NSSA)

  • It is used to allow an ASBR to send external routes through Stub Area by using type 7 LSA.
  • We can filter LSA4 & LSA5, although re-distribution is allowed.
  • ASBR router will generate type 7  LSA, after that ABR will convert type 7 LSA back to type 5 LSA.
  • Default route can be manually configured.
  • Could be implemented on all routers of an area.
  • The default route will come in the form of LSA7.

GNS3Router(router-configure)#area 3 nssa
GNS3Router(router-configure)#area 3 nssa default-information originate

Totally NSSA

  • It filters LSA3, LSA4 and LSA5.
  • The default route will automatically generate.
  • Implemented only on ABR router.
  • It allows re-distribution.
  • Default route originates by ABR in the form of LSA3.
GNS3Router(router-configure)#area 3 nssa no-summary

Different Types of OSPF Routers

In OSPF, routers are categorized into four different types. They are

Internal Router (IR)

Internal Router, as the name suggests have all of their interfaces in the same area.

Area Border Router (ABR)

Area border Router, is the router, which is placed between different area’s. This router connects one or more different OSPF area’s to the backbone area.

Backbone Router (BR)

Backbone Router has at-least one interface in the Backbone Area (i.e Area 0). A backbone router could be an Internal Router (IR).

Autonomous System Border Router (ASBR)

Autonomous System Border Router (ASBR) router is used to provide connectivity between different Autonomous Systems. Thus, one interface of router keeps into OSPF process and the other interface’s is placed different IGP or EGP protocols.

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Summary

In this article, we discussed about Open Shortest Path First (OSPF) routing protocol. It is a great IGP routing protocol. We have also discussed about various Area’s and Routers used in OSPF. Path Cost is the basic metric of OSPF, which is used to calculate the best path to the destination.

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