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Understanding OSPF Neighbor Relationship


The neighbors only forms between routers in the same area.
First thing that happens when ospf forms neighbor as you start the ospf process and the router will determine its own router id


Now lets talk about how OSPF neighbor relationship forms





Step 1. Determin your own Router ID.

·         The router id is simply the router’s name in the ospf process.

The router-id will be:

·         Highest active interface ip address when ospf starts ( loopback beat physical interfaces)

The name is just an identifier for the router
As we get into the advance configuration of ospf that we really need to know the names of your router,  there’s some configuration that you will be typing in that tie directly to the names


Here’s the example
Here's the router down here, it has three interfaces



Now which one becomes the router-id ?

That’s pretty simple.. The highest active ip address is 192.168.1.1, so when we starts the ospf process router will look at the interfaces and say 192.168.1.1 is my name. so from there on now when router speak to other routers that would be its identifier.

If you configure loopback interface then the highest loopback interface would be chosen as the router id.

Sometime its very dangerous to have a change in router-id. But the OSPF router-id only changes when either the ospf processes restarted OR you reboot the router. To restart the ospf processes you nedd to use “clear ip ospf process” command.

You can hard-code the router-id using “router-id” command.


Step 2. Router add the interfaces to the link state database ( Dictated by the NETWORK command )

For this we go to routing process by typing “router ospf 1” command, and then we are going to type in “network” command and what networks we want to run ospf for and that will dictate which interfaces will be included in the ospf process and which interfaces it will decide to send hello message on to find other ospf neighbors, so once it add those interfaces to link state database to its road map of the network it will then perform next step.

Step 3. (R1)Send hello message on chosen interface(s).

This is consider as DOWN STATE.

Router has send a hello message out on its interface but it still waiting to hear back, you will see the networking Is right now considered  DOWN because we haven’t formed any neighbors on there or we should say the neighbors are considered down because no neighbor relationship have been born.

The hello message sent:

·         Once every 10 seconds on broadcast/P-2-P networks

·         Once every 30 seconds on NBMA networks

(Hello message timer is tunable we can tune it lower or higher depending on how much bandwidth we want to eat up with hello messages verses how fast we want to converge if a router consider down AND the dead timer is four times the hello timer So for instance on a broadcast/P-2-P networks its going to send four hello messages before it considers that neighbor is down)

Hello message contains all sort of information:

·         Router ID

·         Hello and Dead timers*

·         Network mask*

·         Area ID*

·         Neighbors

·         Router priority

·         DR/BDR ip address

·         Authentication password*

Now, you might have noticed that some of these have asterisk behind it, anything that you see in the asterisk behind means that the neighbors have to have these thing equal in order to become neighbor.

So R1 first send that hello packet containing all the information to R2


Step 4. (R2) receive the hello.

This is consider INIT STATE.

But after R2 receive hello from R1 it has to check the following things because if any of these are not matching then they can’t become a neighbor.


·         Check Hello / Dead interval

·         Check Netmasks

·         Check Area ID

·         Check Authentication password

And if these things don’t matches then neighbor will be consider as DOWN.

If its all good then R2 will perform following step.


Step 5. (R2) Send replay hello.

This is consider 2-WAY STATE

And that’s the point where R1 & R2 both look at each other’s hello packet and says

·         Am I listed as a neighbor in your hello packet ?

[ If yes, reset dead timer ]

[If no, add as new neighbor ]

If they are already neighbor Step 5 is where the neighbor relationship ends.

If they are new neighbors that’s where we move on to Step 6.


Step 6. Master – Slave relationship determined.

So now lets say the R1 & R2 are new neighbors and follow the process.

Step 6 is where the master and slave relationship determined

This is consider EXSTART STATE (exchange start)

In this state router start exchange of LSDB, master – slave relationship is not a big deal because all it determined is who sends the information first. Whoever is the master will send its LSDB to other router first, and the slave will send it second.

How one router becomes the master:

·         Determined by “priority”, router-id breaks tie

·         Master sends DBD packet (first)

DBD = Cliff(small) notes of link-state database

·         Slave sends its DBD packet (second)


Step 7. DBDs are Acknowledged and reviewed.

This is consider LOADING STATE

During the loading state following process takes place.

·         Slave requests details (Link state Request – LSR)

·         Master sends update (Link state UPDATE – LSU)

·         Master requests details (LSR)

·         Slave sends updates (LSU)


Step 8. Neighbors are synchronized

This is consider FULL STATE

That LSR and LSU process continuous until the neighbors are synchronized.
Now its time to run the DIJKSTRA SPF algorithm to figure out what to do with all this data.

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