Spanning Tree Port Priority and Port ID Values

In this article I am going to discuss about the Spanning Tree Port Priority and Port ID Values so keep reading this article till the end if you want to know about this.

The spanning tree will have a number of switches. And each switches participating in the network have ports and it can be one or more. A value, called Spanning Tree Port Priority Value, is associated with each port of every switch. By default, this value is equal to 128.

Now we are going to learn about Port ID. This ID can be formed by adding two values. They are port priority value(which is 4 bit in length) and an interface identifier(which is 12 bit in length). Therefore, the length of the port ID will be 16 bit.


Spanning  Tree Port ID(16 bit) = Port Priority Value(4 bit) + Interface Identifier(12 bit)

Usually, this Port ID is denoted by using hexadecimal notation other than the decimal notation. Therefore, the hexadecimal notation of the default value of the spanning-tree port id will be similar to the value of 0x8015. Because the equivalent decimal value of 0x8015 is 128.21. Here in this value, the first part represents the, as we already explained, the default port priority number. Likewise, the second part represents the interface identifier of the switch.

The command used for viewing the spanning-tree command is show command. This is written as,

“ show spanning-tree”.

#show spanning-tree


Spanning tree enabled protocol ieee

Root ID    Priority    32769

Address 0001.42AD.E8B3

This bridge is the root.

Hello  Time  4 sec,  Max Age 40 sec,  Forward Delay 30 sec.

Bridge ID  Priority    32769

Address 0001.42AD.E8B3.

Hello Time 4 sec,  Max Age 40 sec,  Forward Delay 30 sec.

Aging Time 40.

Interface Cost        Type Priority Number Status    Role
Fa0/24 19 P2p 128.24 FWD Desg
Gi1/1 4 P2p 128.25 FWD Desg
Gi1/2 4 P2p 128.26 FWD Desg


In the above topology, we can see that, if the bandwidth of the two links is the same, there will occur ambiguity for selecting the Root Switch. Because the Switch Id of the neighbor switches and the Path  Cost of the two paths are the same. To avoid this tie we can use the Port Priority number for tie-breaking.

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In the above case, the Root Port will be the interface on the switch 10. This interface selection is based on the Port Priority number. The root port will be the Port which receives the minimal Port priority number from the switch which is connected to. Here, in this case, this connected switch is the Root Switch. The other port in that switch will be blocked.

I hope you have read and understood about the Spanning Tree Port Priority and Port ID Values.