Open Shortest Path First (OSPF)

Disclaimer: 
===========
All the text written here is sometimes my interpretation of data and sometimes exact words
written in different websites, tutorials, white paper, books and other online and offline sources.
This text is mainly for my personal reference. Thanks.

	- OSPF is a link-state routing protocol.
	- OSPF is an intra-AS or interior gateway protocol. Although it is capable of sending 
	  and receiving routes from other ASes as well.
	- OSPF operates in an heirarchical way. It can have a heirarchy of areas.
	- Routers which have multiple interfaces can participate in multiple areas. Such routers
	  are called Area Border Router (ABR).
	- Topological Database is an overall picture of the networks in relationship to routers.
	  It contains collection of LSAs (Link State Advertisements) received from all the routers
	  in the same area.
	- In an area, all the routers have identical topological databases. A portion of network
	  in which all routers have identical topological databases is also called Domain.
	- Using the concept of areas, OSPF is able to scale well by dividing the network into
	  smaller manageable portions.
	- Area partitioning creates two different types of OSPF routing, depending on whether the
	  source and destination are in same or different areas. 
	  Namely, Intra-Area & Inter-Area routing.

Configuring OSPF:
=================
	- What is process ID? There is a nice link on my homepage about it.
	  Process ID is only to distinguish between multiple OSPF programs running on same router.
	  Which turns out might be required in certain scenarios. But as far as what number to assign
	  to process-ID while starting the ospf router, it doesn't matter. Only thing which matters is
	  that it should be unique within a router. Thats it.
	- In the most basic setup, login to all the routers and execute these commands:
	  1) enable
	  2) config terminal
	  3) router ospf 1234, 1234 is just a random process id chosen and you can use this same id 
	     on all the routers.
	  4) network 10.1.1.0 255.255.255.0 area 0.0.0.0, in this command replace 10.1.1.0 and 255.255.255.0
	     with appropriate network this router is connected to. You can keep rest of the command same.
	     Do it for all the routers and for each router execute this command for all the network it is
	     directly connected to. This means that we are putting all the routers in the same area, area 0.
	  5) end, this exits the ospf router configuration mode and also starts the ospf routing process.
	  6) That's it. You should be able to ping any network from any network.

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