OSPF

Open Shortest Path First (RFC : 2328 )


OSPF States :

1. Down  : No hello packets received , send the hello packet with its own router id.
2. Init : Hello received without routers id.
3.Two way 
4.Ex-start
5.Exchange
6.Loading
7.Full adjacency

OSPF Packet types :


 1.Hello
 2.Database description (DBD)
 3.Link state request (LSR)
 4.Link state update (LSU)
 5.Link state acknowledgement (LSAck)


Used Dijkstra algorithm to calculate shortest path


Link - Interface on the OSPF router

Link State :- IP address / mask of the interface (Link)
                   Metric (cost) of the link
                   Routers connected to that link
                   Type of the network connected to.

Link state database - All the link states will be keep in LSD.

OSPF Steps

• Flooding link state information
• Making a topological table 
• Shortest path first , using Dijkstra Algorithm. This will rearrange the topology.
• From that shortest path tree will make .Router itself as the root.
• Using these information will create routing table.

LSA - Types 

1.Router link entry - Router . In area only
2.Network link entry - DR
3 & 4. Summary link entry - ABR 
5. Autonomous system external link entry - ASBR


Hello packets will multicast to the address 224.0.0.5. 


OSPF Area Types :

• Backbone area
• Standard area
• Stub area
• Totally stubby area
• Not so stubby area (NSSA)

Link 1
Link 2

OSPF network types:

•  Point to point 
•  Broadcast 
• Non Broadcast
• Non Broadcast multi access 
• Point to multi point 

LINK 1