Open Shortest Path First (OSPF) is a link-state routing protocol, rather than a distance vector protocol. The main difference here is that a linked-state 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. Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS). It is defined as OSPF Version 2 in RFC for IPv4.‎Link-state routing protocol · ‎Interior gateway protocol · ‎Intermediate System.


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Routers in area 0 are usually configured in order to aggregate network addresses, so as to propagate network summaries from one area to another one. However aggregation must be specified manually by the operator, in order not to have troubles with network summarization.

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Stub open shortest path first protocol edit ] A normal edge area, besides knowing details about topology of all internal routes within the area itself, imports all external routes from area 0 without any further aggregation. An area is stub when some external routes are replaced by a single default route to reduce the routing information imported from outside: Stub areas are enabled upon an explicit configuration by the network manager: Virtual Link[ edit ] The Virtual Link is a sort of 'tunnel' between two routers, of which at least one belonging to area 0, which logically belongs to area 0, but physically is made up of a sequence of links inside an edge area.

The purpose is to make OSPF believe that those two routers are connected by a fictitious link in area open shortest path first protocol.


Enabling the Virtual Link only requires the area to be crossed only one and Router IDs of the two involved routers, not the IP open shortest path first protocol of their interfaces: Partitioning of an edge area.

Partitioning of area 0. Rather than simply counting the number of router hops between hosts on a network, as RIP does, OSPF bases its path choices on "link states" that take into account additional network information, including IT-assigned cost metrics that give some paths higher assigned costs.

For example, a satellite link may be assigned higher cost than a wireless WAN link, which in turn may be assigned higher cost than a metro Ethernet link. The message specifies the link s for which the requesting device wants more current information.

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They are sent in response to a Link State Request message, and also broadcast or multicast by routers on a regular basis. Their contents are used to update the information in the LSDBs of routers that receive them.

Link State Acknowledgment LSAck Link state acknowledgement messages provide reliability to the link-state exchange process, by explicitly acknowledging receipt of a Link State Update message. An area includes its connecting router having interfaces connected to the network.

open shortest path first protocol

OSPF (Open Shortest Path First)

Each area maintains a separate link state database whose information may be summarized towards the rest of the network by the connecting router. Thus, the topology of an area is unknown outside the area.


This reduces the routing traffic between parts of an autonomous system. Areas are uniquely identified with bit numbers. The area identifiers are commonly written in the dot-decimal notation, familiar from IPv4 addressing.

Open Shortest Path First

However, they are not IP addresses and may duplicate, without conflict, any IPv4 address. When dotted formatting is open shortest path first protocol, most implementations expand area 1 to the area identifier 0. Backbone area[ edit ] The backbone area also known as area 0 or area 0.

All other areas are connected to it, either directly or through other routers.


Inter-area routing happens via routers connected to the backbone area and to their own associated areas. The backbone must be contiguous, but it does not need to be physically contiguous; backbone connectivity can be established and maintained through the configuration of virtual links.

All OSPF areas must connect to the backbone area.

Open Shortest Path First - Wikipedia

This connection, however, can be through a virtual link. For example, assume area 0.

Further assume that area 0. To be a transit area, an area has to have the transit attribute, so it cannot be stubby in any way. Stub area[ open shortest path first protocol ] A stub area is an area which does not receive route advertisements external to the AS and routing from within the area is based entirely on a default route.

This reduces LSDB and routing table size for internal routers.