OSPF routing protocol is 100 percent loop-free protocol. It is the most commons Interior gateway protocol used by most of the organizations. Whether the protocol is using IPv4 or IPv6 it works in the same way in both cases because its metric is bandwidth and bandwidth is considered in layer-1 and layer-2 so regardless of which protocol it uses it works in the same way.

This is the typical network where right in the middle there is a loop but if we use OSPF as it is a Link-state protocol and this protocol makes a storage file called Link-state database, which stores all the routers information in the network all the networks, entire topology of network and information of each and every router which prevents loop.

The negative point of this protocol is that it is resources intensive. The link-state database is stored in the RAM and the topology and map of network are created by CPU and then stored in the RAM, thus this protocol takes more resources, the bigger the network the more resources will be used. The resources are used by all the routers in the network because each router creates its own database.

OSPF uses Dijkstra algorithm which is shortest-path-first (SPF) algorithm. The problem with this algorithm is when there is a change in network topology the router generates the SPF messages and it is sent to all the routers using OSPF and whether the router gets impacted by the change or not each and every router recalculates its information and this is not good because it interrupts the network functionality. If your organization is small then this is not a problem but if it is a growing enterprise with hundreds of routers then it can slow down the network traffic.

For example there is a change in network topology as shown below. This change does not impacts the routers at the very bottom but it impacts the neighbor routers but when this change comes all the router recalculate their database and this takes resources and time.

Area Division

The problem with OSPF where all the router change and recalculate their information is critical for large enterprise and to solve this problem there is a mechanism of Area division.
This mechanism divides the network into areas and the router with the same only talks to each other and they do not take care of the routers outside of area. This segregation helps to prevent the change in all the routers instead the change is only made in few routers.

This is the area division diagram where we have divided a large topology into 3 smaller topologies called area. Now if any change happens in area-1 only the routers belong to this area will update their database and all the other routers will not do anything. However, the ABR will change its database if there is any change in either of its network. For instance discussing above diagram if there is change in area-1 the ABR will make changes to itself and if there is any change in area-0 the ABR will still make changes to its configuration because it is part of two areas.
The routers connecting two different areas are called area border router (ABR). The ABR sits between two areas and transfers the traffic of one area to another. It is important to note that the ABR must be part of at least one zero area (area 0). This concept can be clearly understood using following example.

There we have place an ABR in between area-1 and area-2 but the traffic cannot go directly from area-1 to area-2 because it is mandatory that one area-1 must be area-0 otherwise traffic will not get sent, so if area-1 needs to send the data to area-2 the traffic must go from area-1 to area-0 and then area-0 to area-2.

Remember: At least one area must be area-0. Every area must be connected to area-0.

The topology with multiple areas is called Multi-area OSPF.


Autonomous System Border Router (ASBR) is used to connect to a 3rd party protocol such as Internet or any other router protocol such as EIGRP. For instance you have your OSPF set and you want to connect to the Internet then the area you will use to connect to the Internet (or any protocol except OSPF) is called ASBR, it connects to different protocols.
Area Border Router (ABR) connects to area in the OSPF protocol.

OSPF Working

We talked earlier that the metric of OSPF is bandwidth but it is not true the true metric is cost and this cost is calculated using bandwidth. The path that has more bandwidth it means it has less cost and the best route.

This is the formula and cost is calculated between two routers and then overall cost is aggregated. The single cost is turned around to next digit and aggregated cost is not. We need to take a scenario to evaluate the cost.

This is the network with two bandwidths.

We convert the bandwidth in bits measurement. Notice that the numbers rounded up no matter how small number is after point, the number will always be rounded up.

This 195 is the total cost of the path. The more bandwidth the less cost it gets.

OSPF works fine with low bandwidth as it makes accurate calculation, but with high bandwidth it goes worse. We consider three different bandwidths 100 Mb, 1 Gb, and 10 Gb and OSPF calculates the path cost for all three as 1.

OSPF rounds up the number and they all get 1 and this is problem with high bandwidth which is discussed in CCNP to solve this. This problem with OSPF considers all higher links with different bandwidth equally and this is a critical issue. This becomes more critical when we encounter this problem with WAN but fortunately we do not this and other IGP in WAN connection.

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