What is OSPF (Open Shortest Path First) in Networking?

OSPF (Open Shortest Path First) is a Link State routing protocol defined in RFC 2328.

What is OSPF (Open Shortest Path First) in Networking?

What is OSPF Protocol?

The Link State routing protocols know the entire network map. Triggered Updates are also sent to ensure fast network integration when there are any changes to the network.

On a network using the OSPF protocol, Routers send link status notifications to each other when a change occurs, for example when a new Router is added to the network environment or a Router fails.

In case of any change in the network, the affected Router sends the current LSA packets to the network environment. Other Routers update the network topology with the LSA packets it receives. It also refreshes the SPF trees and updates the routing tables.

This protocol uses the Dijkstra algorithm and sends updates every 30 minutes. It also supports IPv4 and IPv6, such as EIGRP. It uses a value called Area and divides large networks into areas, reducing the size of the routing table and the complexity of the network.

Maintains a high level of hardware requirements such as RAM and CPU. Configuring or editing this routing protocol used in large networks requires expert personal.

Features

If we list the OSPF features;

1. Uses Area and Autonomous System.
2. Minimizes routing updates.
3. Supports CIDR and VLSM.
4. Provides reliability.
5. Unlimited Hop Count feature.
6. It can also be used on non-Cisco devices.
7. The AD (Administrative Distance) value is 110.
8. Provides route authentication.
9. It provides fast integration.
10. Sends updates only when the network changes.
11. It does not send updates to the entire routing table.
12. Uses the SPF algorithm to calculate the lowest cost to a target.
13. Sends Hello packets to neighboring Routers every 10 seconds.
14. In non-broadcast networks, the Hello packet sending time is 30 seconds.

Routing Tables

1. Neighbor Table

It stores the information of the neighboring devices that the router owns in this table. Each Router has its own neighbor table. Each router has an IP address and interfaces information for neighboring devices.

2. Link State Table

This table holds connection status information for neighboring devices. The routing table on the other Routers holds the link status information and each Router has a Link State Table.

3. Routing Table

This table holds the metric of each record followed by the Link State table. As a result, it keeps the route information for the shortest route.

OSPF and RIP Difference

CharacteristicsOSPFRIPv2RIPv1
ProtocolLink StateDistance VectorDistance Vector
ClasslessYesYesNo
VLSM SupportYesYesNo
Auto-SummarizationNoYesYes
Manual SummarizationYesNoNo
DiscontiguousYesYesNo
Route PropagationIt sends multicast when the network changes.It sends the regular multicast.It sends the regular broadcast.
Path MetricBandwidthHopsHops
Hop Count LimitUnlimited1515
ConvergenceFastSlowSlow
Peer AuthenticationYesYesNo
Hierarchical Network RequirementYes (using Area)NoNo
UpdatesTriggeredRoute TablesRoute Tables
Route ComputationDijkstraBellman-FordBellman-Ford

When you look at the image above, you can see that Router R1 is in the Area0 area, and it is a Backbone Router. And so you can communicate with routers that are connected to different areas.

Network Topology

When OSPF network topology designs, Area must determine. The OSPF routing protocol must have an Area 0 and all other OSPF Areas must be connected to this Area. As an example of OSPF network topology, you can examine the following image.

Backbone, ABR ve ASBR Router

When we look at the above image, Router R1 is in Area 0 field and is a Backbone Router. Routers connected to different OSPF areas can communicate with each other.

Metric Calculation

Metrik hesaplama işlemi bantgenişliğine göre yapılmaktadır. Metrik hesaplanırken her bir bağlantının bantgenişliği hesaba katılır.

Cisco Router metriğini varsayılan olarak 100 Mbps olarak ayarlar ve buna göre değerlendirir.

64-Kbps (64,536-bits-per-second) link:100,000,000 / 64,536= 1,562
1.544-Mbps (T1) link:100,000,000 / 1,544,000= 64
10-Mbps link:100,000,000 / 10,000,000= 10
100-Mbps link:100,000,000 / 100,000,000= 1
1-Gbps link:100,000,000 / 1,000,000,000= 0.1
10-Gbps link:100,000,000 / 10,000,000,000= 0.01

Terminology / Terms

Link: Link is the interface associated with a network.

Router ID: An IP address used to identify the router. In the OSPF network, each Router must have RID. By default, the largest IP address is selected as the RID. To determine the router ID, Loopback interfaces are usually defined.

Neighbor: A neighbor is a point-to-point connection of one or two Cisco Routers.

Adjacency: Route information updates are shared directly between the two Routers. Unlike EIGRP, route information is shared directly between neighboring Routers.

Hello Protocol: Uses Hello packets to establish and maintain a neighbor relationship between routers. Hello packages and LSA packages build and maintain the logical database.

Neighborship DB: A list of all Routers. Router ID and State information are stored and maintained in the neighborhood database.

Topological DB: A database created by LSA packets received from an Area. It is used as a record for the Dijkstra algorithm used to find the shortest path.

Link State Advertisement (LSA): A data package containing link status and routing information.

Designated Router (DR): A DR Router collects routing information in itself and distributes it to other Routers.

Backup Designated Router: When the DR Router is not active, the BDR Router is enabled. It receives all routing information from other neighboring devices but does not send LSA packets.

Area: Area is a grouping of adjacent networks and routers. All routers in the same domain share a common Area ID. The Area ID is associated with a specific interface on the router. Routers within the same Area have the same topology table.

Broadcast (multi-access): A multi-access network consisting of a group of Routers. DR and BDR Routers must be selected in the broadcast network.

Non-Broadcast (multi-access): Non-Broadcast networks. For example; Frame-Relay and ATM are a network of Non-Broadcast.

Point-to-Point: A network type that provides a direct connection between two Routers that provide a single communication path.

Point-to-Multipoint: The type of communication on a router with the connection of multiple Router groups.

Configuration 1 ⇒ Video

Using Packet Tracer, you can watch the video below for routing on Routers and also subscribe to our YouTube channel!

Configuration 2 ⇒ Video

You can also watch the following video to configure it with the GNS3 program!

   Final Word


In this article, we have provided information about OSPF, a popular dynamic routing protocol. Thanks for following us!

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