Introduction to OSPF

OSPF is a we all know is a link-state routing protocol. All routers within an OSPF area maintain a complete topology information of that area. A particular interface on a router can be part of only a single area which can lead to some issues if the network isn’t designed correctly.

In this post we will have a look at a scenario where we will use the OSPF multi-area adjacency feature. OSPF Multi-area adjacency forms more than one adjacency over single link. This feature only works on the OSPF point-point links. Let us use the topology below to understand further:

Related- OSPF Neighbor States

OSPF Multi-area Adjacency

In the below topology we will try to reach 1.1.1.1/32 network from R5.

Requirement is to prefer the path R5 > R4 > R3 > R2 > R1 and not via path R5 > R4 > R2 > R1.

We have configured the network diagram below as follows:

R1 – R2: Static routing
R2 – R3: OSPF Neighbors in Area 0
R2 – R4: OSPF neighbors in Area 10
R3 – R4: OSPF neighbors in Area 10
R4 – R5: OSPF neighbors in Area 10

Under the normal OSPF configuration we will see that the path R5 > R4 > R2 > R1 is preferred over the path R5 > R4 > R3 > R2 > R1 which doesn’t meet our requirement.

Configurations and Output as below:

Trace to 1.1.1.1/32 network from R5 takes path via R4 > R2.

R5#traceroute 1.1.1.1

R4#sh ip route 1.1.1.1

We see leaving everything to default in OSPF the route towards 1.1.1.1 from R5 is preferred via R4 > R2 rather than going from R3 as we have lower cost/metric via R2 which is 21 and from R3 it will come out to be 22 due to extra-link in network.  Default metric of ASBR is 20.

Now let us try to change the OSPF cost on R4 connecting to R2 and try tracing the path again. As in output below we see R4 still installs the route to 1.1.1.1 via R2 even when the cost is higher than that from R3.

R4#sh ip route 1.1.1.1

Trace output still is the same traversing via R4 > R2.

R5#traceroute 1.1.1.1

Once we shut the link towards R2 we see the route to 1.1.1.1 is installed via R3 and trace also starts going via R3.

R4#sh ip route 1.1.1.1

R5#traceroute 1.1.1.1

The reason behind such behavior is that information needed to reach the ASBR is inter-area information however intra-area information to reach the ASBR is always preferred irrespective of the OSPF cost via two paths.One way to resolve this behavior is to use the OSPF multi-area Adjacency.

Multi-Area ADJ can be used in order to form multiple point-to-point logical adjacencies over a single link. The requirement is that there must be only two OSPF speakers on the link, and in a broadcast network, you must manually change the OSPF network type to Point-to-Point on the link. This feature allows a single physical link to be shared by multiple Areas and creates an Intra-Area path in each of the Areas that share the link.

Configuration on R2 and R3 as below to enable OSPF Multi-Area ADJ.

R2:

R3:

Continue Reading:

Basic OSPF Configuration of HP Switches

OSPF Cost Calculation

ABOUT THE AUTHOR

Rashmi Bhardwaj

I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”

I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.

I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)