# Introduction to Spanning-Tree | NetworkLessons.com

**URL:** <https://forum.networklessons.com/t/introduction-to-spanning-tree-networklessons-com/3466>\
**Category:** Spanning Tree\
**Created:** [April 6, 2018, 6:03pm UTC](https://forum.networklessons.com/t/introduction-to-spanning-tree-networklessons-com/3466 "2018-04-06T18:03:43Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![kingsnetworks3](https://cdn-forum.networklessons.com/letter_avatar_proxy/v4/letter/k/b5a626/32.png) [@kingsnetworks3](https://forum.networklessons.com/u/kingsnetworks3)\
**Post date:** [April 6, 2018, 6:03pm UTC](https://forum.networklessons.com/t/introduction-to-spanning-tree-networklessons-com/3466/1 "2018-04-06T18:03:43Z")

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> **[Introduction to Spanning Tree](https://networklessons.com/spanning-tree/introduction-to-spanning-tree)**
>
> This lesson is an introduction to spanning-tree. You will learn how spanning-tree works and how you can verify the spanning-tree config on Cisco switches.

Hi I have one doubt why Sw3 is port priorities are showing 128 + port number, actually the port ID = default port priority + Interface number + 2 right ..?

```
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/14 Desg FWD 19 128.14 P2p 
Fa0/16 Desg FWD 19 128.16 P2p

```

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<div class="post-metadata">

**Author:** ![lagapidis](https://cdn-forum.networklessons.com/user_avatar/forum.networklessons.com/lagapidis/32/4949_2.png) [@lagapidis](https://forum.networklessons.com/u/lagapidis)\
**Post date:** [April 10, 2018, 3:24pm UTC](https://forum.networklessons.com/t/introduction-to-spanning-tree-networklessons-com/3466/2 "2018-04-10T15:24:12Z")

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Hello Networks

The port priority is defined as the port priority + the interface number. The port priority is 128 by default, but can be adjusted anywhere from 0 to 240 in increments of 16 (0, 16, 32, 48 etc). The interface number is composed of 12 bits so this number can be anywhere between 0 and 4096. This means that a switch with up to 4096 ports can be acomodated.

From this information, the port priority is always priority.port. This means that by default, the priority of port FastEthernet 0/14 will always be 128.14.

This is defined in section 9.2.7 Encoding of Port Identifiers on the 802.1D-2004 specification and can also be found in this [Cisco command reference](https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_55_se/commmand/reference/3750cr/cli3.html#91880).

I hope this has been helpful!

Laz