CCNA RnS Article #32: Influencing STP

 ⭐CCNA RnS Article #32: Influencing STP⭐

One of the aspirants asked a question and I think is worth to write on this in this article. The question was is there a way we can change the default behavior of the STP? I mean the default settings work fine for a controlled small network. But in large networks, administrators need control over which switch should be a root and which port should be in a forwarding or blocking state.

STP Topology @www.thenetworkdna.com
Fig 1.1- STP Topology

Default settings on Cisco switches work well for some small networks. For instance, all switches that have default Bridge ID (BID) that is a combination of default priority value (numeric value) and adding universal MAC address that is assigned to hardware during manufacturing. Another example is the cost associated with a port – this cost is based on the operating speed NOT on the maximum speed of the interface. For example, a 10/100/1000 interface that due to some reason working on 100 Mbps, will get the cost of 100 Mbps speed not for 1000 Mbps speed.

With this explanation, there are only two options for network administrators to play with if they want to influence the STP topology.

To change the BID of the switch, the network engineer can set the priority used by the switch and leave the MAC address as it is. For example, giving the switch the lowest priority value among all the switches will result in that switch winning the root switch election.

You need to understand the default cost values, that you can configure on the ports to impact other switches’ calculation of the root cost. For example, to favour one link over another, assign a lower cost or to avoid a link for forwarding state, give a port a higher cost.

Link Speed (Operational)


10 Mbps


100 Mbps


1000 Mbps


10 Gbps


100 Gbps


1 Tbps


Cost values in the above table are the latest IEEE cost: 2004 and after. But if you are a veteran, your values were different when there were no high-speed interface requirements and rapid convergence. Hope you find this informative!

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