What is OSPF Protocol in Networking? – OSPF (Open Shortest Path First) is a Link State routing protocol as defined in RFC 2328.
What is OSPF Protocol in Networking?
What is OSPF Protocol in Networking?
Link State routing protocols know the complete network map. In addition, when there is a change in the network, Triggered Updates are sent to provide rapid network integration.
In a network using the OSPF protocol, OSPF-configured routers send connection status notifications to each other when a change occurs, for example when a new router is added to the network environment or when a router fails.
When occurs any changes on the network, the Router affected by the change will send the current LSA packets to the network environment. The other routers update the network topology with the received LSA packets. It also updates the SPF trees and updates the routing tables.
OSPF uses the Dijkstra algorithm and sends updates every 30 minutes. OSPF supports IPv4 and IPv6, such as EIGRP. The OSPF protocol also uses a value called Area. OSPF areas are divided into areas in large networks, reducing the size of the routing table and the complexity of the network.
OSPF keeps hardware requirements like RAM and CPU at a high level. Configuring or organizing the OSPF routing protocol used in large networks requires an expert person.
If we list the OSPF features;
1. It uses Area and Autonomous System.
2. It minimizes routing updates.
3. It supports CIDR and VLSM.
4. It provides reliability.
5. It has the Unlimited Hop Count feature.
6. It can also be used on non-Cisco devices.
7. AD (Administrative Distance) is 110.
8. It provides Route Authentication.
9. It provides fast convergence.
10. It sends updates only when the network changes.
11. It does not send updates about all of the routing tables.
12. It uses the SPF algorithm to calculate the lowest cost to a destination.
13. It sends Hello packets every 10 seconds to neighboring routers.
14. On non-broadcast networks, the time to send the Hello package is 30 seconds.
OSPF Routing Tables
OSPF routing tables are divided into 3. These;
1. Neighbor Table
This table stores information about neighboring devices that the router has. Each Router running OSPF has its own neighboring table. Each Router has IP address information and interfaces information of neighboring devices.
2. Link State Table
This table holds the connection status information of the neighboring devices. The routing table on the other Routers maintains link state information. It has each Router Link State Table that uses the OSPF protocol.
3. Routing Table
This table holds the metric value of each record tracked by the Link State Table. As a result, it maintains route information for the shortest route.
OSPF and RIP Difference
You can compare OSPF and RIP in the following table.
|Protocol||Link State||Distance Vector||Distance Vector|
|Route Propagation||It sends multicast when the network changes.||It sends the regular multicast.||It sends the regular broadcast.|
|Hop Count Limit||Unlimited||15||15|
|Hierarchical Network Requirement||Yes (using Area)||No||No|
|Updates||Triggered||Route Tables||Route Tables|
By dividing a large network into small areas with the OSPF protocol, you can have a faster, reliable, scalable and powerful network.
OSPF 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.
When we look at the above image, Router R1 is in Area0 field and is a Backbone Router. Routers connected to different OSPF areas can communicate with each other.
How Calculate OSPF Cost?
The OSPF metric calculation process is based on bandwidth. When the OSPF metric is calculated in Cisco Routers, the bandwidth of each link is joined to the bandwidth account.
The Cisco Router sets the OSPF metric to 100 Mbps by default and evaluates accordingly. The following table shows the default OSPF metrics for various bandwidths on Cisco routers.
|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|
OSPF Terminology / Terms
After discussing what is OSPF, let’s talk about OSPF terminology or OSPF terms to better understand the OSPF protocol.
Link: Link is the interface associated with a network.
Router ID: The Router ID (RID) is an IP address used to identify the Router. OSPF network must have each Router RID. By default, the largest IP address is selected as the RID. To determine the Router ID, Loopback interfaces are usually defined.
Neighbor: Neighborhood is a point-to-point connection between two Cisco routers.
Adjacency: Route information updates between two OSPF routers are shared directly. Unlike EIGRP, route information is shared directly between neighboring routers.
Hello Protocol: The OSPF protocol uses Hello packets to establish neighbor relationships between routers and maintain the relationship. Hello packages and LSA packages build and maintain the logical database.
Neighborship DB: A list of all OSPF routers. Router ID and State information are kept and protected in the neighborhood database.
Topological DB: It is a DB created by LSA packages from an OSPF Area. It is used as a record for the Dijkstra algorithm used to find the shortest path.
Link State Advertisement (LSA): An OSPF data packet containing link state and routing information.
Designated Router (DR): A DR collects Router routing information itself and distributes it to other Routers.
Backup Designated Router (BDR): If the DR Router is not active, the BDR Router enters the circuit. Neighborhood receives all routing information from other established devices but does not send LSA packets.
OSPF Area: OSPF Area is a grouping of adjacent networks and routers. All routers in the same area share a common Area ID. The Area ID is associated with a specific interface on the router. The routers in the same Area have the same topology table. When configuring the OSPF configuration, it is important to keep in mind that the Area 0 Backbone Area configuration.
Broadcast (multi-access): It is a multi-access network consisting of a group of OSPF Routers. DR and BDR Routers must be selected in the broadcast network.
Non-Broadcast (multi-access): It refers to non-broadcast networks. For example; Frame-Relay and ATM are Non-Broadcast Networks.
Point-to-Point: It is a network type that provides a direct connection between two Routers providing a single communication path.
Point-to-Multipoint: The Interface on a Router is the type of communication with the connection that is created by more than one Router Group.
The above OSPF terminology or terms will give you a good understanding of what is OSPF and what it does.
How to Configure OSPF Protocol in Cisco Packet Tracer? ⇒ Video
How to Configure OSPF Protocol in GNS3? ⇒ Video
Also, you can watch how to configure OSPF Protocol in GNS3 to better understand OSPF in networking…
What is OSPF Protocol in Networking? – We have explained about the OSPF features in detail. In our next tutorial, we will configure OSPF configuration in detail with network simulator programs.
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