What is OSPF – Link State Routing Protocol

OSPF

• Open Shortest Path First.
• Link-state Routing Protocol.
• Each OSPF has a Processor ID- its just a Identity.
• Metric used for OSPF is Bandwidth
• Lower the Metric is better.
• Metric = Reference bandwidth / Interface bandwidth
• OSPF works based on Areas, and the neighbors are formed within the same area.
• Area 0 is said to be as Back-bone Area. Every Area should be connected to backbone area(Area 0).
• OSPF will generate LSA(Link state Advertisement), which is status of the connected links.
• All routers with in the area share their LSA’s , so they could build their LSDB(Link-state database).
• LSDB is nothing but complete routing information about OSPF networks.

DR and BDR

• Consider you are having 10 routers and each routers are interlinked with each other(Fully Mesh).
• In that case, they would send traffic to all routers, which will result in having a lot of OSPF traffic.
• In order to avoid that, OSPF uses something called DR(Designated Router) and BDR(Backup Designated Router)
• DR and BDR will be elected only in Multi-access network
• DR and BDR is not based on Areas, and it is based on Network
• DR (Designated Router)
• All traffic from routers will be send to DR only
• DR will reply to all other routers.
• BDR (Backup designated Router)
• BDR is the Backup of DR.
• If DR crashes, BDR will takes place.

How DR and BDR Elected. ?

• DR and BDR is elected based on Priority.
• By default, Every Router has a Priority of 1.
• The router with the Highest Priority will be Elected as DR and the one with next will be Elected as BDR.
• If the Router has the Priority of 0, will never participate in the DR and BDR Election, hence will not never Become DR or BDR.
• If the OSPF Priority is same on all the Routers in the Network, then Router id is the tie-breaker.
• The router with highest Router id will become DR and the next will be BDR.

Areas

• Routers in the Backbone Areas (Area 0) are called as Backbone routers.
• Every Area must be connected to the Area 0, otherwise Routing will not occur.
• Routers between 2 areas (like the one between area 0 and area 1) are called Area-Border Routers (ABR). In other words, the Router that has more than one area Configured is said be as ABR.
• Routers that has OSPF configured and other Routing Protocols as RIP, EIGRP … are configured, such routers are said to be as Autonomous System Border Router (ASBR).

OSPF commands

• Router-1#show ip ospf neighbor
• It gives you information about Neighbor Router-id, OSPF State(DR or BDR), Local Interface to which it is connected.
• Router-1#show ip route ospf
• It gives the information about the Network, Local Interface, AD value and Metric.
• Router-1#show ip ospf interface f0/1
• This gives you information about the cost of the interface, the Network type, Router ID, Process ID, Area details.

That's it about OSPF 

You can also look into my video tutorial below:

Introduction to OSPF – Link State Routing Protocol || [TAMIL]

Introduction to OSPF – Link State Routing Protocol || [ENGLISH]

If you have questions, Kindly comment below.