19 - OSPF - CBT CCNP VID 10 - STATES

Question 1

((Down State))

After starting the OSPF process on the router,

Answer 1

(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

(second step)
After adding the interfaces with the relavent ip addresses using the network command,
these will be added into the Link state Database.

( only with devies within the same OSPF area )

(third step)

• Sends hello 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 hello timer. 10sec x 4 = 40 & 30sec x 4 = 2mins

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

Question 2

((Init state))

(STEP FOUR)
After sending a multicast hello message - It receives a unicast hello message from a router that want to become a neighbour

Answer 2

• 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 3

((two way state))

(step five)
- am I listed as a neighbour in your hellos packet ?

Answer 3

-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 4

((Exstart state))

(Step six)

Answer 4

(Step six)
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 5

((Loading state))

Step seven

Answer 5

((Loading state))
(Step seven)
-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 6

((Full state))

Answer 6

(Step eight)

• 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.