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OSPF is the link state protocol which uses flooding of link state information and Dijkrstra least cost path algorithm. It develops complete topological map with the help of OSPF and shortest-path in the autonomous system by running Dijkrastra’s algorithm on the router. In OSPF, router broadcasts routing information to all the other routers in the autonomous system. The router broadcast the link state information whenever there is change of link cost. It broadcast the link’s sate periodically at least once every 30 minutes. The OSPF protocol checks that links are operational by sending HELLO message that is sent to the attached neighbors. It allows the OSPF router to obtain a neighboring router’s database of network-wide link state.
Some of the advantages of OSPF are as follows:
- The specification is available in the published literature. As it is open standard anyone can implement it without paying the license fees that encourage many vendors to support OSPF.
- Load balancing is performed by OSPF. If there are multiple routes with the same cost then OSPF distributes traffic over all routes equally.
- OSPF allows site to partition networks and routers into subnets called area. The area topology is hidden from other area and each area is self-contained. The area can change its internal topology independently thus it permits growth and make the networks at a site easier to manage.
- All the exchanges between the routers are authenticated. OSPF allows variety of authentication schemes. Different area can use different authentication scheme. Router authentications are done because only trusted routers should propagate routing information.
- There is an integrated support for unicast and multicast routing. Multiple OSPF (MOSPF) provides multicast routing. MOSPF is simple extension to OSPF. MOSPF uses existing OSPF link database and adds a new type of link-state advertisement to the existing OSPF link-state broadcast mechanism.
- OSPF support for the hierarchy within a single routing algorithm.
Hierarchical structure of OSPF network is shown below:
[ Diagram similar to diagram from kurose]
- Internal router - They are within an area. They only perform intra AS routing.
- Area border router - These router belong to both an area and the backbone.
- attached to multiple areas
- runs a copy of SPF for each attached area
- relays topological info on attached areas to backbone
- Backbone router - These routes perform routing within the backbone but themselves are not area border routers.
- AS boundary routers – A boundary router perform inter-AS routing. It is through such a boundary router that other routers learn about paths to external networks.