OSPF — The basics you should know

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“Open Shortest Path First” has been the most popular routing algorithm after RIP (Routing Information Protocol) became obsolete.

The Should Knows

• A link-state routing protocol
• Classless routing protocol — Supports subnetted networks
• Fast convergence — quickly propagate to network changes
• Secure — supports MD5 authentication, but should be enables
• Efficient — trigger updates
• Uses Dijkstra Shortest Path First (SPF) algorithm to find the shortest path

Data structures

1. Adjacency database
   • Creates the neighbor table
   • The neighbor table contains a list of neighboring networks
2. Link-state database
   • Creates the topology table
   • The topology table contains a list of all possible routes to all known networks within the area
3. Forwarding database
   • Creates the routing table
   • The routing table consists of the best route to all the known nodes in the area

Routing Protocol Messages

1. Hello Packet
   • Used to discover, build and maintain neighbor adjacencies.
   • To establish an adjacency both ends should agree with parameters in the hello packets
   • Multicasted periodically
2. Database Description Packet (DBD)
   • Used to synchronize the link state Db between routers
   • After adjacencies are formed, a DBD packet is sent to describe the Link state DB so that the routers can compare whether the DBs are in sync
3. Link-State Request (LSR)
   • After sending DBD packets, routers might identify that they are not having an up-to-date DB
   • LSR is used to request for the most recent LSDB
4. Link-State Update (LSU)
   • The reply to LSR packet
   • A unicast only to the requester
5. Link-State Acknowledgement (LSAck)
   • A reply to the LSU
   • Used to make flooding of LSAs more reliable.
   • LSA receiver send this packet to the LSA sender to inform that the LSA has been received properly

OSPF Operation

Following are the steps in the OSPF operation followed by and image:

1. Establish neighbor adjacencies by using Hello packets
2. Exchange LSAs — The LSA’s are not LSAcks. LSAs will be explained in a future article
3. Build the topology table
4. Execute the SPF algorithm — also in a later article

Basic operation of OSPF

These are the should knows before digging deep into the OSPF concepts and configurations.