What is OSPF? - Open Shortest Path First Protocol

Open Shortest Path First (OSPF) is a link-state routing protocol which is used to find the best path between the source and the destination router using its own Shortest Path First). OSPF is developed by Internet Engineering Task Force (IETF) as one of the Interior Gateway Protocol (IGP), also OSPF (Open Shortest Path First) protocol is known as one of a family of IP Routing protocols, and is an Interior Gateway Protocol (IGP) for the Internet, used to distribute IP routing information throughout a single Autonomous System (AS) in an IP network.

It is a network layer protocol which works on the protocol number 89 and uses AD value 110. OSPF uses multicast address 224.0.0.5 for normal communication and 224.0.0.6 for update to designated router (DR)/Backup Designated Router (BDR).

The OSPF protocol is a link-state routing protocol, which means that the routers exchange topology information with their nearest neighbors. The topology information is flooded throughout the AS, so that every router within the AS has a complete picture of the topology of the AS. This picture is then used to calculate end-to-end paths through the AS, normally using a variant of the Dijkstra algorithm. Therefore, in a link-state routing protocol, the next hop address to which data is forwarded is determined by choosing the best end-to-end path to the eventual destination.

The main benefits of a link state routing protocol like OSPF is that the complete knowledge of topology allows routers to calculate routes that satisfy particular criteria. This can be useful for traffic engineering purposes, where routes can be constrained to meet particular quality of service requirements.

The main minus point as we say disadvantage of a link state routing protocol is that it does not scale well as more routers are added to the routing domain. Increasing the number of routers increases the size and frequency of the topology updates, and also the length of time it takes to calculate end-to-end routes. This lack of scalability means that a link state routing protocol is unsuitable for routing across the Internet at large, which is the reason why IGPs only route traffic within a single AS.

OSPF router transfers information about its local state to other routers usable Link State Advertisement message. Every routers uses the received messages to build up an identical database that describes a topology of Autonomous System.

Using the database each and every router calculates own routing table using Shortest Path First (SPF). This routing database contains all destination of the routing protocol know about based on the next hop IP Address and interface.

Important points of OSPF Protocol

• OSPF protocol recollect routes whenever network topology have changes, using the Dijkstra algorithm, and minimises the routing protocol traffic that it generates.
• Protocol provides a support for multiple paths of equal cost.
• Protocol provides a multi-level hierarchy (two-level for OSPF) called "area routing," so that information about the topology within a defined area of the AS is hidden from routers outside this area. This enables an additional level of routing protection and a reduction in routing protocol traffic.
  
• All protocol exchanges can be authenticated so that only trusted routers can join in the routing exchanges for the AS.

Version 3 OSPF

OSPF version 2 is used and allow only for IPv4 address, but this OSPF v3 is allowed both as IPv4 and IPv6’s 128-bit address space. However it’s not a only change but this is the major change in between of OSPF v2 and v3.

It supports and allow the protocol processing per-link instead of per-subnet.

Addition of flooding scope, which may be link-local, area or AS-wide

Remove the opaque LSAs

Support for multiple instances of OSPF according to the link

Various packet and LSA format changes (including removal of addressing semantics).

Both OSPFv2 and OSPFv3 are fully supported by DC-OSPF.

OSPF basic Commands

Once the OSPF is configured, let’s look at a few basic OSPF commands available on Cisco IOS and similar industry-standard CLIs.

The command “show ip ospf neighbor” displays OSPF neighbors and their state. In this case, we see R1 and R2 fully adjacent to each other via their GigabitEthernet 2 interfaces.

The neighbor ID equals the router ID of the neighbor.

The priority is related to the election of a designated router — not important for our simple example.

On a point-to-point link, the OSPF state should be “full.” If it’s not, something has probably gone wrong.

The dead time is a countdown timer constantly being reset as messages are heard from the neighbor. If the dead time gets to zero, the neighbor is presumed dead, the adjacency is torn down, and the link removed from SPF calculations in the OSPF database.