Spanning Tree Protocol Part 2


Once again this is our architecture and in this lecture we will learn how to change the priority number to change the root bridge.


This is lab design where we are working.

Types of Roles

In STP the ports are assigned to 3 roles.

·       Designated is the port which is allowed to forward traffic.

·       Root is the port closer to the root bridge and cannot be shut down.

·       Non-designated/Alternative is the port which is not allowed to forward the traffic and kept shut down.




Cisco supports two spanning tree protocol standards 802.1d and 802.1w which is Rapid spanning tree protocol. Rapid spanning tree protocol does not take 30 seconds to turn off or on the port it does it instantly.
If you want to know which spanning tree you are using whether regular or rapid you can check his by looking at the following line.


If there says spanning tree enabled protocol rstp it means it is rapid mode, in our switches it is regular.

Types of Stats

There are 4 status of spanning tree protocol, the status tells whether ports traffic is being forwarded, blocked or any other type.

·       Blocking/BLK is blocking state where data is blocked.

·       Forwarding/FWD is forwarding state where data is forwarder.

·       Listening/LIS is the state which takes 15 seconds it blocks the port as soon as new cable is plugged and listens for BPDU to decide further action.

·       Learning/LRN also takes 15 seconds and it decides whether new plugged port is creating any loop or not and then the decision is made whether to make the status FWD or BLK.

·       Disabled is the only state which is not visible in configuration and it means that cable is not plugged in.

The total 30 seconds are divided into two parts the first 15 seconds are for Listening and last 15 seconds are for Learning.


Root Port Selection

The port closer to the root port is selected by using many techniques, the very first is bandwidth which is 19 if the bandwidth is 100Mbps, and is 4 if the bandwidth is 1Gbps. If bandwidth ties then priority number is tested and if ties as well then interface number is tested which one has lower number is assigned as closer to root.

Changing Root Bridge

We can change the root bridge to make our ideal switch root. This is done by decreasing the priority number and it can be done using two methods.


This is the one way where we make the switch root and it gets it priority number decreased.


The other way is by changing the priority number manually. The number could be from 0 to 61440, but it is increment of 4096 not 1.


Notice that when we entered 111 it did not accept because it only can assign the above 16 numbers to root.


Now we have setup root as our core router but this is not all, the election still takes place among other devices and if you really want to make a managed network you should set the priority number by hand.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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