Types of Routers in OSPF

• Internal router - Router with all interfaces in the same area.
• Backbone Router/ Backbone internal router - Router who’s all interfaces are in the same area and that area is back bone area.
• Area Border Router - Router which provides connectivity between two areas and make possible for them to communicate with each other.
• Autonomous System Boundary Router - Router which provides connectivity between two autonomous system and make possible for them to communicate with each other.

IR	Internal Router
ABR	Area Border Router
BBIR	Backbone Internal Router
ASBR	Autonomous System Boundary Router

DR, BDR, IR, BIR, ABR, and ASBR a router can have multiple states at the same time.

§  Topology is physical arrangement of devices

Three steps are involved in convergence.

I. Neignbor table. (Contains details of directly connected neighbors)

II. Topology table. (Contains picture of complete network, all routes in network)

III. Routing table. (Contains the best path to reach any network)

Neighbor table

It is made when OSPF process starts and routers send LSAs to their directly connected neighbors

Neighbor table is different on each and every router, it contains details of directly connected neighbors.

Database/ Topology table

Also known as topology table, it contains all possible paths for reaching a destination, it contains details about all the devices and complete picture of network, database table is same on all routers.

Example

If you take a snap of this room and distribute this picture to all so every person in this room will see the same map of the class 

Routing Table

After algorithm is applied on database table to find the best path to a destination (Every routing protocol have its own algorithm OSPF uses dijkstra algorithm) It is also different on each router.

It is not possible for a router to make topology table without making neighbor table, similarly it’s not possible for router to make routing table without making topology table, these are sequential steps.

Convergence is when information on all the routers has been advertised or shared to all other devices in network.

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