Introduction to OSPF
Open Shortest Path First (OSPF) is one of the Interior Gateway Protocol (IGP) which helps to find the best routing path between the source and the destination router using its own shortest path first (SPF) algorithm. It is a Link state routing protocol which is used to distribute routing information about data packets within a large Autonomous System.
OSPF Areas
An autonomous system can be divided into areas, these help in reducing the link state advertisements and other overhead traffic that will be otherwise sent to the network.
Let’s take a look:
- Backbone Area: It is also known as area0 or area 0.0.0.0 it forms the very core of an OSPF network, and all other areas of the network are connected to the backbone area. It is responsible for distributing routing information between areas that are not backbone area types.
- Stub Area: In the case of Stud Area, routing in the area is entirely on the basis of a default route. It is an area which does not receive advertisements external to the autonomous system (AS)
- Not so Stubby Areas: NSSA is a type of stub that is able to import AS external routes and send them to any other area. However, it is unable to receive AS external routes from other areas of the network.
- Transit Areas: It is an area with 2 or more OSPF border routers and it can be used to pass network traffic from one adjacent area to another one. It does not originate traffic and nether it is the destination of any such traffic.
How OSPF Works?
When it is configured, it listens to its neighbors in the networks and it gathers all the link state data available. This data is then used to make a topology map that contains all available paths in the network. This database is saved for use and we call it Link State Database.
Once the Link State Database is made it is used to calculate the shortest path to subnets/networks using an algorithm known as Shortest Path First which was developed by Edsger W Dijkstra. OSPF creates 3 tables:
- Routing Table: It contains currently working best paths that will be used to forward traffic between two neighbors.
- Neighbor Table: This contains all discovered Open Short Path First neighbors.
- Topology Table: This one contains the entire road map of the network. This road map includes all the available Open Short Path First routers and keeps calculated data about best and alternative paths.
Router Types in OSPF
- Internal Router: This router contains all interfaces that belong to each other in the same area.
- Area Border Router: ABR connects one or more areas with a backbone network. An ABR is considered a member of all the areas it is connected to. It keeps multiple Link State databases in the memory, one for each area.
- Backbone Router: A router that has an interface to a backbone area is called a backbone router.
- Autonomous System Boundary Router: ASBR is a router that is connected to the network with more than one routing protocol. ASBR exchanges routing info with routers autonomous systems. These run an exterior routing protocol, use stating routes or even use both methods.
Applications of Open Shortest Path First
OSPF is the first widely deployed routing protocol. It can converge with a network in a few seconds and it is one of the protocols that can provide loop-free paths. Aside from these features, Open Short Path First allows the imposition of policies for the propagation of routes in the network.
Open Short Path First is better at load sharing on external links compared to other IGPs. Considering these benefits, it can found widespread use.
Implementations of Open Short Path First
- Microsoft’s Windows NT 4.0 Server, Windows 2000 Server and Windows Server 2003 all have OSPF v2 in the Routing and Remote Access Services. Microsoft Removed the support with the Windows Server 2008 and later version of Server Operating Systems by the company.
- OpenBSD Operating system has an implementation of OpenBGPD protocol which has OpenOSPFD implementation.
- BIRD implements OSPFv2 and OSPFv3 both.
- GNU Zebra is a GPL routing suite that supports OSPF for Unix-Like systems.
- Multi-Protocol Routing module in Netware has support for OSPF.
Advantages and Disadvantages of OSPF
Below are the advantages and disadvantages:
Advantages
Below are the advantages:
- Open Short Path First is easily scalable, meaning with the very little amount of hassle, we can scale it to use in a very big network
- Open Shortest Path First Protocol has full support for subnets.
- Use of Hello Packets: Open Short Path First sends small hello packets to verify link operations and ignores transferring large tables.
- OSPF Supports route tagging: In Open Short Path First, routes can be tagged to ease interoperation with arbitrary values.
- Routing: Open Short Path First is able to route packets based on their type of service field.
Disadvantages
Below are the disadvantages:
- It is a processor intensive protocol to use.
- Because it maintains more than one copy of routing information, it consumes more memory.
- It is a more complex protocol to understand and learn compared to other Internet Protocols.
Conclusion
Open Shortest Path First as a routing protocol has an important place in internet infrastructure. Being able to find the shortest path easily and quickly helps in reducing unnecessary network load and the ability to find another path in case of error at the optimal one helps in increasing the stability of the network.
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