Saturday, November 14, 2015

Drawing OSPF Road Map Manually

Ever wondered how an OSPF Link State Graph or Shortest Path Tree (SPT) will look like? Well this is just modeling the Road Map which can be drawn using the Link State Database of an OSPF process. This is how the router uses self-originated and received LSAs to calculate the best path via a graph.
Download the configuration files from here.

Let's think as the router. He only understand what other routers say via LSAs. I am taking a random router named as R3 (router-id: in a lab network.

Starting form viewing the LSDB which has all the LSAs he can see that there are 6 routers in the OSPF process numbered as 1 in the area 0 which he is also in.

(click on the images for full view)

The the SPT is drawn using the Router LSAs.

1st he sees what His own Router LSA says to others.

It says that there are 2 transit networks connected to him. Transit networks are network segments.

Their DRs are and and the Cost is 10 for the routers in those segments.

So the road map from him will start like this.

 Now it's the time to query Network LSAs to find out what those DRs are adjacent or connected with.

Network LSAs say that those DRs are connected to & nodes as well.

So he can continue drawing his map.

It's time to query the Router LSAs received from & to figure out their neighbors, attached links and costs.

Now he can continue drawing his map using these information like before.

Now he queries Network LSAs of new 3 DRs to find out other routers in those segments.

Now the map will be like below.

Now he queries other Router LSAs of & & to find out their networks.

Here this router has a stub network with ip address of

Its a loopback.

The new DR ( found from these Router LSAs is connected to & only. Following is its Network LSA.

Now the final diagram will be updated like this.

This is the road map which is used to reach & calculate the costs to all routes in the network. Every router in the same area has a copy of this. But the actual costs in the routing table will be like below.

It will be confusing if we look at the map and try to compare the costs of the routing table to the routes. Actually there is no problem in the map. Its is confusing because of the DRs in the map. They are used to calculate the costs to the network segments only. Because the Network LSAs say which routers are advertising it we can draw a simplified map by removing the DRs now. Then it will be ok.

By the way, the actual physical topology was like this.

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