What Is OSPF Protocol ? OSPF Basics And Overview

ospf lsa types

OSPF ( Open shortest path first ) is a dynamic routing protocol which is used for routing. In Networkel we talked about EIGRP basics before, it is time to take a look “what is OSPF” and “how OSPF works”OSPF ( Open shortest path first ) is a link – state protocol like IS-IS which we will examine later. All the other routing protocols are distance vector protocol.

Like we talked about before in EIGRP, OSPF too keeps some routing informations on its tables. These tables are neighbor table, topology table and routing table.

OSPF Neighbor Table

OSPF neighbor table keeps the OSPF neighbor relationship like EIGRP neighbor table.

OSPF Topology Table

OSPF topology table is much different from the “topology table” in EIGRP. OSPF using routers, keep the path map for all networks in the area that belongs too. For example if you want to go from Paris to Marseille, Google Maps shows us the best path in spite of it has all the paths between Paris to Marseille ! ( In this example best path is routing table ) . But the topology table that is used in EIGRP, keeps the information that the neighbor router tells.

OSPF Routing Table

OSPF routing table keeps the best path for the target network.

OSPF ( Open shortest path first ) uses the SPF algorithm as you can understand from its name too. This algorithm consumes much CPU so you should be careful when you are using. In this algorithm, routing tables aren’t sent once at 30 seconds as RIP. Just update messages are sent if there is a change in the network. Additionally , LS refresh messages are sent in long periods ( 30 minutes ). This message incluedes all routing table of a link state router and sent for keep the network secure.


ospf design

  • As you can see from the figure, we have a router named ABR. ABR is Area Border Router. ABR’s one interface should be in Area 0, other interfaces may belong to any other areas. That creates a border between areas. For example if a change occurs in network at Area 2 , ABR localizes it and doesn’t send to other areas.
  • ASBR ( Autonomus System Boundary Router ) is used for connecting to different ASs.
  • All areas should be connected with Area 0.
  • All routers in the same area have the same topology table.
  • Requires hierarchical design.
  • Just ABR and ASBR can make route summarization in OSPF.


ospf cost

As you know, routing protocols create “cost” values according to their algorithms and find the best path targetting to other networks. In OSPF  cost is calculated like 100 / Bandwdith ( Mbps ). First, all routers should equalize their topology tables, then they start to make SPF algorithm run.For example when R1 is ready to make SPF work, put itself to center of everything tries to find the best routes in all network. In OSPF , there is no feasible successor like in EIGRP. If a network is down, SPF algorithm is run again and best path is re-calculated.

DR (Designated Router ) AND BDR ( Backup Designated Router )

Let’s what is designated router ( DR ) and backup designated router (BDR). When you run OSPF there are two types of router which are designated router ( DR ) and backup designated router (BDR). This selection is made to decrease the unnecessary traffic in our OSPF network. For example if a router loses a network, it sends this information to just DR and BDR over DR takes this info and forwards it to all other neighbors over DR and BDR selection is made for each shared segment. Selection is made due to priority value of the routers which is defaultly 1. ( We can hard code and change it ). Router 1 establishes a full state neighborship with just DR and BDR routers. It establishes a 2 way state neighborship with the other routers.

OSPF Packet Types

There are six OSPF packet types :

1 Hello
2 Database description (DBD) (Contains link-state advertisement (LSA) headers only and describe the contents of the entire link-state database )
3 Link-state Request (Request for networks that is not in the routing table)
Link-state update (Reply packet for the link-state request packet)
LSA (Link state advertisement) = Individual route informations in LSU packet.
LSACK (Lick State Acknowledement) = Acknowledge message which is sent for all messages except “hello”.

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