OSPF (Open Shortest Path First)

Next to EIGRP there is one more IGP (Interior Gateway Protocol) we are going to look at
which happens to be OSPF (Open Shortest Path First). This first chapter will be an
introduction. Some of the things you might have seen from CCNA but I’ve added some
extras.
Link-state routing protocols are like your navigation system, they have a complete map of
the network. If you have a full map of the network you can just calculate the shortest path
to all the different destinations out there. This is cool because if you know about all the
different paths it’s impossible to get a loop since you know everything! The downside is that
this is more CPU intensive than a distance vector routing protocol. It’s just like your
navigation system…if you calculate a route from New York to Los Angeles it’s going to take a
bit longer than when you calculate a route from one street to another street in the same
city.

Features and advantage of OSPF

  • It supports both IPv4 and IPv6 routed protocols.
  • It supports load balancing with equal cost routes for same destination.
  • Since it is based on open standards, it will run on most routers.
  • It provides a loop free topology using SPF algorithm.
  • It is a classless protocol.
  • It supports VLSM and route summarization.
  • It supports unlimited hop counts.
  • It scales enterprise size network easily with area concept.
  • It supports trigger updates for fast convergence. 
  • Multicast addressing 224.0.0.5  /   224.0.0.6 
  • It is a layer 3 prtocol work on network layer 
  • IP protocol  no 89
  • Administrative Distance (AD) 110 
  • Manual summrization on ABR & ASBR router
  • Authentication plain text & MD5
  • Update trigged 

    OSPF table :-

    1 Neighbor table :- Each router discovers its neighbors on each interface, list kept in neighbors table.

    2 Database table :- Each router then runs the SPF algorithm against its own topology database to calculate the best routes to each subnet in the database.

    3 Routing table:- Each router finally places the best route to each subnet in the IP routing table.

     

    OSPF LSA:-
     
    Let’s take a good look at link-state and what it exactly means: 
    Link: That’s the interface of our router.
    State: Description of the interface and how it’s connected to neighbor routers.
    Link-state routing protocols operate by sending link-state advertisements (LSA) to all other link-state routers.     

     

    OSPF LSDB:-
 All the routers need to have these link-state advertisements so they can build their linkstatedatabase or LSDB.
Basically all the link-state advertisements are a piece of the puzzle which builds the LSDB.

This LSDB is our full picture of the network, in network termswe call this the topology.




 OSPF AREA TYPES :-

 1 BACKBONE AREA
2 INTER AREA
3 EXTERNAL AREA

 1 BACKBONE AREA :- Backbone is the central point of this implementation. Routers running in this area required to maintain a complete database of entire network. All areas need to connect with this area through a physical link or via a virtual link if physical link is not possible.
OSPF BACKBONE AREA 0
2 INTER AREA :- IR (Internal Router) is a router running in area off backbone. IR only needs to maintain an area centric local database.

3 EXTERNAL AREA :- the extension of backbone. Routes running in this area required to maintain an area specific database instead of complete database. This is a cool feature. It will speed-up the convergence time.

Basic OSPF Configuration