Port or Interface - Which Should You Use in the Context of STP?

[promanelka]

While studying about the Spanning Tree Protocol and how it works, I kept on seeing the words, ports and interfaces used. I wondered: "Why use both in two different sentences if they actually mean the same thing?" I thought they meant the same thing until I did some research into how the word "interface" is used when describing VLANs and explaining Layer 2 frame switching. Here's what I found:

1. Interfaces can be Ethernet, VLAN, etc. and they mostly refer to logical or physical connection point on a network. I'll like to think of it as the intangible or software part of that connecting point where you insert an Ethernet link/cable.
2. Port is the physical connection point on a switch or router. I like to think of it as that classic shape of the RJ-45 or any other Ethernet cable into which a link can be inserted. That is why ports are more related to Spanning Tree Protocol as they refer to the physical connection set to block or forward frames.

The Rule:

• In CLI (when configuring), use the term interface.
• When discussing STP states or roles, use the term port.

[M02@rt37]

Hello @promanelka 

STP operates at layer-2, where switches forward frame based on @MAC. STP is concerned with physical connectivity and preventing loops, so it makes sense to use "port" because it directly refers to the switch's physical forwarding decisions.

In CLI configuration, the term interface is used because you are configuring a specific network connection, whether physical or logical...

[promanelka]

Thanks a lot for your additional insight. It makes it a lot clearer. 🩵

[Ramblin Tech]

Your working definitions are quite reasonable and not inconsistent with common industry usage.

IEEE 802.1D (and the superseding 802.1Q) uses "bridge port" when referring to the component of a bridge that receives and transmits frames to and from a LAN. Cisco generally uses "interface" when referring to the XE/XR/NXOS configurable entity that connects the switch/router to some transport medium. When appropriately configured, a Cisco "interface" can be an IEEE "bridge port".

Stay flexible when using port vs interface and keep the context in mind. TCP and UDP both make use of "ports" which are strictly software constructs and not "physical connection points". Likewise, an L1 "interface" has definitions for the physical connector and optical/electrical characteristics that might also be called a "port".

[promanelka]

I'm definitely going to take note of this. Thanks a lot for your clarification. 🩵

[MHM Cisco World]

In simple words

Physical Port-interface in simple SW 

Physical Port-multi interface in SP router 

That why we use port or interface in SW but we use only interface in SP routers

MHM

[promanelka]

Hmmm... Now I know. Thanks @MHM Cisco World 🩵

[MHM Cisco World]

You are welcome 

MHM