CCNA OSPF Database and Networks


Chapter #1 – The OSPF Database

OSPF maintains a database of topology information, called the Link State Database (LSDB). This includes routers (identified by Router-ID), and the links between them.

We can look at the contents of the LSDB with show ip ospf database. The router link states are the routers, and the net link states are the links between the routers.

A router will advertise its links with an LSA (Link State Advertisement). The LSA is a data structure within a packet that the router sends to a neighbour.

DBDs (Database descriptors) contain summaries of the database entries. They are sent to neighbours, so the neighbour can see what they’re missing. If a neighbour is missing anything, it will request more information about the specific entries with an LSR (Link State Request).

A Link State Update (LSU) is the response with the requested information. This is acknowledges with an LSAck message.

Once the LSDB is built, OSPF will run the Shortest Path First (SPF) algorithm to find the best paths through the network.

Chapter #2 – Network Types

There are several OSPF network types. For CCNA, we’re interested in:

  • Broadcast
  • Point-to-point

Point-to-point networks are used between two routers only. There can be no possibility of any more routers on that network segment.

The broadcast network type is for ethernet. This is where multiple routers could be on  a VLAN.

The problem with broadcast networks, is that LSA flooding could generate too much traffic. So, for broadcast networks, OSPF elects a router to be the Designated Router (DR) and the Backup Designated Router (BDR). These routers handle the LSA flooding, to avoid all routers flooding to all other routers.

Chapter #3 – Neighbour States

There are several states neighbours go through when they are forming adjacencies

Down This is where the process starts. You generally wouldn’t see a router here for long, unless there’s something wrong

Attempt This is only used in NBMA networks. This is where unicast messages are sent to neighbours. This can be ignored for the CCNA exam.

Init A hello packet was received from a neighbour, but the local Router-ID was not included. This is where a neighbour is announcing its Router-ID, but hasn’t learned the local Router’s ID yet.

2-Way Hello messages are passing between two neighbours, and they see their own Router IDs in the hello messages. This is a complete bi-directional adjacency. At this point, DR and BDR routers are elected. Any DROthers stay in the 2-Way state.

ExStart The remaining states focus on synchronizing information between the neighbors. In the ExStart state, the router with the highest Router ID is chosen to be master. This router initiates the sync process.

Exchange DBD packets, that is summaries of the LSDB entries, are shared between neighbours. A router will look at the LSA information in the DBD to see if it’s missing anything.

Loading If there is topology information missing, a router will request more information with LSR messages. Neighbours will provide this information with LSU messages, which are then acknowledged with LSAck messages.

Once the database has been built, each router runs the SPF algorithm to find paths through the network.

Full This is the (other) normal state for routers. The network has fully converged, and routers will just maintain their databases at this point.

Command Summary

show ip ospf databaseGlobal ExecShows the contents of the LSDB
show ip ospf neighborGlobal ExecShows all OSPF neighbours
ip ospf priority VALUEInterface configSets the OSPF DR priority on an interface
clear ip ospf processGlobal ExecClears OSPF, including neighbours
ip ospf network TYPEInterface ConfigSets the network type on an interface
debug ip ospf adjGlobal ExecDebug OSPF adjacencies (neighbours)