Mastering Cisco OSPF: Understanding Link State Database, Network Types, and Neighbor State

In this video, we will be learning more about OSPF.
We’re going to cover:
0:00 The OSPF Database
3:53 Network Types
13:45 Neighbour States

If you missed Part 1 of our OSPF CCNA training, you can find it here -    • Demystifying OSPF: How It Works and Connec...  

OSPF routers keep a full map of the network topology. Unlike other routing protocols, they don’t just have ‘signposts’ pointing the way, they see the whole thing.

They store topology information in the Link State Database (LSDB). This is populated with information that they learn from their neighbors. This includes Router-IDs (Router Link States) and links (Net Link States).

To share this information, they send first send DBDs (Database Descriptors), containing LSAs (Link State Advertisements) to each other. These are summaries of the database. Another router can request more information with an LSR (Link State Request). It will receive a Link State Update (LSU) in response, and finally acknowledge it with an LSAck.

Once the database has been populated with information, the SPF (Shortest Path First) algorithm is run, and OSPF works out the best paths through the network.

OSPF has four different network types. The two we need to know for CCNA are Broadcast, and Point-to-point. Point-to-point is used when there will only be two routers on a link. Broadcast is used with Ethernet, when there could be many routers on the link.

To keep updates on a broadcast network to a minimum, one router per segment (VLAN) is elected to be the DR, and another is the BDR. These handle distributing OSPF updates to other routers on the segment (called DROthers).

OSPF routers use multicast for their messages. 224.0.0.5 is for all OSPF routers, while 224.0.0.6 is for all DR or BDRs.

While neighbours are forming, they go through several states. These are:
Down - The start of the process
Attempt - Only for an NBMA network (not needed for CCNA)
Init - Hello messages have been sent
2-Way - Bidirectional communication established, DR and BDR are assigned
ExStart - A Master router is selected
Exchange - DBD packets exchanged
Loading - LSRs and LSUs build the database
Full - The process is finished


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