Introduction to Open Shortest Path First (OSPF) Protocol

The Open Shortest Path First (OSPF) protocol is a link state protocol that handles routing for IP traffic. Its newest implementation, version 2, which is explained in RFC 2328, is an open standard. Open Shortest Path First (OSPF) is an open standard (not proprietary) and it will run on most routers independent of make. Open Shortest Path First (OSPF) uses the Shortest Path First (SPF) algorithm, developed by Dijkstra, to provide a loop-free topology. Open Shortest Path First (OSPF) provides fast convergence with triggered, incremental updates via Link State Advertisements (LSAs). Open Shortest Path First (OSPF) is a classless protocol and allows for a hierarchical design with VLSM and route summarization

The main disadvantages of Open Shortest Path First (OSPF) are Open Shortest Path First (OSPF) requires more memory to hold the adjacency (list of OSPF neighbors), topology (a link state database containing all of the routers and their routes), and routing tables, Open Shortest Path First (OSPF) requires extra CPU processing to run the SPF algorithm and Open Shortest Path First (OSPF) is a complex routing protocol.

The two important concepts in case of OSPF are Autonomous Systems and Areas. Areas are used to provide hierarchical routing, within an Autonomous System. Areas are used to control when and how much routing information is shared across your network.

OSPF implements a two-layer hierarchy: the backbone (Area 0) and areas off of the backbone (Areas 1–65,535). Here the two different areas can summarize routing information between them. Route summerization helps to compact the routing tables. All areas should connect to Area 0 and all routers in an Area will have the same topology table.

Open Shortest Path First (OSPF) metric value

Open Shortest Path First (OSPF) uses cost as the value of metric and uses a reference bandwidth of 100 Mbps for cost calculation. The formula to calculate the cost is reference bandwidth divided by interface bandwidth. For example, in the case of Ethernet, it is 100 Mbps / 10 Mbps = 10.

Important Terms related with Open Shortest Path First (OSPF)

Router ID

Each router in an OSPF network needs a unique ID. The ID is used to provide a unique identity to the OSPF router.

• The highest IP address on its loopback interfaces (this is a logical interface on a router)

• The highest IP address on its active interfaces

What is a Loopback Interface

A loopback interface is a logical, virtual interface on a router. By default, the router doesn’t have any loopback interfaces, but they can easily be created. These interfaces are treated as physical interfaces on a router and we can assign ip addresses to them.

Router(Config)#int loopback 2
Router(Config-if)#ip address 200.0.0.10 255.255.255.0

Area border router (ABR)

An Area border router (ABR) is a router that connects one or more OSPF areas to the main backbone network. It is considered a member of all areas it is connected to.

Internal router

An Internal Router is a router that has only OSPF neighbour relationships with routers in the same area.

Backbone router

Backbone Routers are part of the OSPF backbone. This includes all area border routers and also routers connecting different areas.

Designated Router (DR) and Backup Designated Router (BDR)

A Designated Router (DR) is the router interface elected among all routers on a network segment, and Backup designated (BDR) is a backup for the Designated Router (DR). Designated Routers (DRs) are used for reducing network traffic by providing a source for routing updates. The Designated Router (DR) maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in an area will form a slave/master relationship with the Designated Router (DR).