OSPF Step-by-Step

understand basic OSPF routing protocol and how its functions and what type of algorithm it uses to select best routs

OSPF Basics

• Open standard devloped by IETF
• Establishes Adjacency with routers 
• Sends Link State Advertisments LSA's to other routers in an area
• Constructs a links state database from received LSA's 
• Runs Dijkstra Shortest Path First for algorithm to determine the shortest path to a network
• Attempts to inject the best path for each network into a router's routing table

OSPF Terminology 

• Hello: a protocol used to discover OSPF neighbors and confirm reachability to chose neighbors also used in elections of designated routers.
• Link State Advertisement (LSA): information a router sends and receives about network reachability, used to construct a router 's link state database.
• Links State Update LSU: a packet that carries LSA's 
• Link State Request LSR: used by a router to request specific LSA information from a neighbor
• Link State Acknowledgement LSACK: used bu router to confirm it received LSU

DR and BDR

Adjacencies only to be formed with DR and BDR

How DR gets Elected ?

• Hello protocol used to elect a DR
• During DR election, router with highest priority value wins.
• OSPF priority value is associated with an interface and can be a value in range 0 - 255
• An OSPF priority value 0 means that the router will not become a DR
• The default Priority value of an interface is 1
• If the priorities tie, the router with highest router ID (RID) becomes DR
• A router ID can be configured in router configuration mode, with command router-id value
• If an RID is not configured, the highest IP address of a loopback interface becomes the RID
• If a router no loopback interface, the highest IP address of non-loopback interface becomes the RID

Types of LSA

OSPF Example with 2 Areas