Category Archives: ospf

OSPF route withdraw

OSPF has slightly different way of removing routes compared to BGP. On this short post, I will present how a link failure is propagated to other routers on OSPF domain. For this test, I have the following topology section in which AREA3 is connected to AREA0 and we simulate a link failure on the Junos router J39 which has the subnet


Before the failure, we can see that is contained in router LSA.

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OSPF Loop prevention

On this post, I will show an example of loop prevention on OSPF protocol. There is a nice document at here about the principles of loop prevention. What I will just do is to show this on Junos. In order to show this, I am using the following topology;


On this topology, J40 and J32 are ABRs. J40 has a connection to Area3 in broadcast segment and J32 also has a link to Area3 via J201. It looks like we have multiple paths towards the same Area3 from backbone area. Let’s see how OSPF handles this.

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OSPF equal cost path

On this post, I will try to show how OSPF behaves when there are two equal cost paths towards a destination. To demonstrate this, I have prepared my usual topology.


On this topology all routers are running OSPF but our focus is on the router J32 which is circled at the bottom and our destination network is Since we have set the reference bandwidth to 10G and each links are 1G a single link OSPF cost is 10. Now let’s see how J32 device reaches this destination network.

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OSPF neighborship, database and routing table

OSPF sometimes can be a confusing protocol. For example if you turn on a light switch, you simply get the immediate result: Light is on. What if when you turn on the light switch but you see the shining light bulb after 40 seconds then it is more difficult to understand the result of your actions. This was a small introduction to this post which I will try to write about he link between OSPF neighborship, OSPF database and routing table. All tests are done on a virtual SRX firewall in packet mode. I have cropped a section of my OSPF topology to simplify this post.


We simply have two routers connected via a point to point link and they both run OSPF. FF31 router is an ABR and J204 is an internal router (which is actually a routing instance on another router but we can ignore this detail)

On this setup following are the OSPF details

J204: Router ID=, Interface IP:
J31: Router ID=, Interface IP:

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