19 - OSPF - CBT CCNP VID 10 - steps of nieghbour relationship

Question 1

(first step)

(Down State)

Answer 1

After starting the OSPF process on the router,

(first step)
It chooses the router ID. Router ID is the router Name in OSPF.
It is how routers using ospf identifies each other.
* By default they way the router chooses its name is
- Highest active Interface Ip address when OSPF starts

Hierarchy Router prioritises when choosing the Ospf ID:

• Manually entering the router ID
• Highest Loopback interface Ip address
• Highest Physical interface Ip address

Router id is important because
- ID is used To configure Ospf
- if Router interface are added or remove, all config using previous router iD is useless, as the ID has just changed.
This is why hardcoding the router iD is important.

Router ID only changes if:
- ospf process is restarted
- router is restarted
E.g if Ospf ID is not hardcoded, then if the router reboots weeks later, the neighbour relationship breaks!

Question 2

(second step)

Answer 2

After adding the interfaces with the relevant ip addresses using the network command,
these will be added into the Link state Database.

Question 3

(third step)
Neighbour process.
——————-
( only with devies within the same OSPF area )

Answer 3

• Sends hellow messages on chosen interfaces
• once in every 10 seconds on broadcast & point to point networks
• once in every 30 seconds on NBMA network
• dead Timer is 4 times the hellow timer. 10sec x 4 = 40 & 30sec x 4 = 2mins

All the above hellow timers & dead timers can be tweaked to make it faster

Whats inside the hellow message ?
Hellow message is like profile with all its OSPF details

• Hellow & dead Timers
• Network Mask
• Area ID
• Authentication pasword

THE ABOVE ALL HAVE TO BE THE SAME ON BOTH ROUTER THAT ARE TRYING TO FORM A NIEGHBOR RELATIONSHIP

• Router ID
• DR/BDR ip address
• router priority
• Neighbours

Question 4

(STEP FOUR)

(Init state)

Answer 4

After sending a multicast hello message - It receives a unicast hello message from a router that want to become a neighbor

• how do we know its a knew neighbour and not an exisiting one ? It does not have this router in its neighbour table & it respond via unicast message
• This is why this stage is called the initialisation stage - officially known as the ‘init state’
• In this init state the router checks to make sure neighbour details below are the same or it wont add each other as a neighbour
• If the details do not match it will repeatedly go from “init state” to “Down state” - back and forth

These details in the hello message must be the same

• Hello & dead Timers
• Network Mask
• Area ID
• Authentication password

Question 5

(step five)

(two way state)

Answer 5

• am I listed as a neighbour in your hellos packet ?
  -if yes - reset the dead timer
  -if no - add as a new neighbour
  Formed a neighbour relationship
  They are formally in each others database as neighbours

Question 6

(Step six)

(Exstart state)

Answer 6

Let’s Start the exchange of each other’s database.

• But before we do let decide who the master to go first.
• Master - highest router priority decide who is the master.
• If the router priority is a tie the highest Ospf router ID breaks the tie

-Master then sends database description packet (DBD).
A Summed up version of network routes, coz it might already know them.

• slave then sends it’s (DBD) (via reliable connection)

Question 7

(Step seven)

(Loading state)

Answer 7

• DBDs are acknowledged and reviewed.
• if routes are not know they will be requested, by Slave first
• slave will request first ( LSR )
• master send updates ( LSU )
• master request details ( LSR )
• slave send updates ( LSU )

Question 8

(Step eight)

(Full state)

Answer 8

• when router finished sending routes and both devices have the exact same link state database (DBD) (routing table)
• neighbours are fully synchronised !

Now the router runs the DIJKSTRA SPF algorithm to figure out what to do with all this Data.