Welcome back for part two. Neighbor relationships. This is an important part before we dive into discussing the details of exchanging routers with LSA’s in part III. Before we start that we need to know how neighbors for and what happens area wide amongst routers.

Before we dig down deeper I am going to put up a list of acronyms to help the various components of OSPF piece together.


  • LinkState Data Base (LSDB) – database held by router – stores the topology table.
  • Shortest Path First (SPF)  – Algorithm used by OSPF to analyse the LSDB. Determines best,       lowest cost router for each prefix
  • LinkState Update (LSU) – OSPF message that holds detailed topology information(LSA)
  • LinkState Advertisement (LSA)  – OSPF data structures that hold detailed topology information.      LSA’s are held in the memory inside LSDB. They are communicated inside LSU’s
  • Area – Contiguous collection of routers. Actively learn all topology      information within an area.
  • Area Border Router  – Has interfaces connected to two or more areas. One interface must be connected to the backbone. Holds topology data for each area it  is connected too. Calculates and advertises routes between areas
  • Backbone Router (BB)   – Any router with an interface in area 0.
  • Internal Router (INT) – Only has interfaces in one area
  • Designated Router (DR) – Elected by OSPF to generate area wide LSA’s
  • Backup Designated Router  (BDR) – Is a backup DR if DR fails.

Let us start with the Database Exchange Process.


  • Hello – Discovers neighbours. Has values that allow neighbour-ship, brings to a 2-way state and monitors link status
  • Database Descriptor – Exchange brief versions of each LinkState Advertisement during initial topology. Allows router to know list of neighbour’s known LinkState Advertisments.
  • LinkState Request – Lists LinkState ID of Link State Advertisement sender of the LinkState Request would the like recipient of LinkState Request to supply during exchange.
  • LinkState Update – Contains fully detailed LinkState Advertisement. Response to LinkState Request
  • LinkState ACKnowledgement – Confirms receipt of LinkState Update


When starting OSPF on a router and assigning interfaces to participate in the process the following occurs.

  • Down – No hellos before dead interval expires
  • Attempt – Neighbour defined with neighbour command.After hello TX before RX
  • Init – Hello Received. No local RID or does not pass verification check.
  • 2Way – Hello Received. RID and local neighbour checks pass.
  • ExStart – Negotiating DBD sequence numbers. Uses Master/Slave logic.
  • Exchange – Finished negotiating. Exchanging DBD packets
  • Loading – DBD exchanged. Sending LSR, LSU and LSAck. Exchange full LSAs
  • Full – Neighbours are fully adjacent. LSDBs for that area are identical.


What is a Designated router?

A designated router is the router that is unique to an area. OSPF uses a DR in an area for two reasons. One is to create and flood Type 2 network LSA’s for that subnet. The second is to aid in the process of a database exchange in the subnet. An election for a router to become a DR takes place based on the information in the OSPF hello messages. Hello’s list the priority and the RID. The election rules are as follows

  • Choose the router with the highest priority
  • If tied, highest RID
  • The BDR is the next highest

The DR adopts the pseduonode ip address of All routers in the area send their Type 1 LSA’s and their ROUTER LSA’s to this node. The Type 2 network LSA which the DR sends out is composed of the ROUTER LSA’s that are connected to it. The DR creates that T2 LSA for the subnet. The DR assigns the LinkState ID of the DR’s interface IP in that subnet. It also lists the DR’s RID as the router advertising the LSA.

R1         R2


R3         R4

x = pseudonode – emits Type 2 LSA

When a DR and a BDR exist, no elections are made until one failts. If a DR fails then the BDR is elected the DR. This is the case even if a better DR joins the network. A new BDR is elected to fill the place. If a BDR fails the DR remains the same then only a new BDR is elected.


Phwoar. A lot to start with. As we go through feel free to cross reference posts. I do appreciate the feedback as well as my peers calling me out if there are mistakes. I am no expert but this is a way for me to reinforce what I know. I will do a whole post on LSA’s and the exchanging of routes next. Then we can configure some routers and check some output. Theory before the lab makes the lab gel better!


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