Spanning Tree Protocol Explained | Step by Step

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Spanning Tree Protocol / STP is a big topic on the CCNA exam. Many people struggle with STP so in this video, we try to break it down as much as possible. Spanning Tree follows a strict set of rules which when learnt, will allow you to answer any STP questions you get.

This video focus on the standard 802.1d (and PVST+) which is the original STP standard.

Election Process

1. Elect a Root Bridge
Switch with the lowest BID

2. Place all Root Bridge interfaces in to a Forwarding state

3. Each Non-Root switch elects a Root Port
Lowest cost
Lowest neighbour BID
Lowest neighbour port priority
Lowest neighbour port number

4. Elect a Designated Port
Lowest root cost
Lowest BID
Lowest neighbour port priority
Lowest neighbour port number

5. All other ports are placed in to a Blocking State

#ccna #cisco #networking

[goddoesskate]

Just in case this is helpful for others, here are my notes from this video.

STP Overview

Spanning Tree Protocol

Types of STP
- Original STP
- STP / 802.1D
- PVST+
- Cisco improvement adding a per VLAN feature
- Cisco default
- RSTP / 802.1w
- Improved STP with much faster convergence
- Rapid PVST+
- Cisco improvement of RSTP adding per VLAN feature
- Makes a large network more efficient

Why STP?
- STP is used to prevent loops when using redundant switches
- Broadcast messages are sent all the time and Broadcast Storms are easy to trigger
- Loops also cause unstable MAC address tables because they're constantly being changed/updated
- Duplicate frames are being sent to the same host

How STP Works
- Switches in a loop 'drop' one of the ports
- Switch with the blocked port still receives the data but it ignores it
- Simple but how the switches choose the port to block can be tricky

Choosing the blocked port
- 1) Elect a root bridge
- King of switches
- 2) Place root bridge interfaces into a Forwarding state
- 3) Each non-root switch selects its Root Port
- This is the best route to the bridge
- 4) Remaining links choose a Designated Port
- 5) All other ports are put into a Blocking state

Roles
- Root Ports
- The best port to reach the Root Bridge
- Designated Port
- Port with the best route to the Root Bridge on a link
- Non-Designated Ports
- All other ports that are in a blocking state

States
- Disabled
- Port that is shutdown
- Blocking
- A port that is blocking traffic
- Must move to Listening state before moving to Forwarding
- Listening
- Not forwarding traffic and not learning MAC addresses
- Transitional State while changing from 1 role to another
- Held in this state for the Forward Delay timer (15 sec default)
- Learning
- Not forwarding traffic but learning MAC addresses
- Transitional State while changing from 1 role to another
- Held in this state for the Forward Delay timer (15 sec default)
- After this, the port can now move to a Forwarding state
- Forwarding
- Sending and receiving traffic like normal
- Can move directly to Blocking

Root Bridge Election
- Each switch has a BPDU. BPDU contains:
- Root Cost
- Cost of the root bridge
- BID (Bridge ID)
- The switch with the lowest overall BID will become the root bridge
- They look something like: 32769aaaa:aaaa:aaaa
- BID is made up of:
- STP priority
Default value of 32768 + VLAN number
For VLAN 1, the STP priority would be 32769
- MAC Address
- Each switch thinks it should be the root bridge
- They share their BPDUs with each other
- Once they all agree, the root bridge has been elected
- All ports on the root bridge enter a Forwarding State
- Each non-root switch will now choose the best path to the root bridge
- This is the Root Port
- This is based on Port Cost
- Cost is based on port speed
- Better speed, lower cost
- Each outgoing port to the root added together
- Can be set manually
- If a tie happens, they look at lowest neighbor BID
- If they tie, they use lowest neighbor port priority
If they tie, lowest neighbor port number
- Select a Designated Port
- Look at lowest root cost to the bridge
- If that ties, lowest BID
- If that ties, lowest neighbor port priority
If that ties, lowest neighbor port number
- Every port that is not a root port of designated port is put in a Blocking State

Downfall of STP -- Convergence
- The time it takes to do the work and become stable

Timers - Default (RSTP addresses the delay of convergence)
- Hello
- Every 2 seconds
- Lets everyone know everything is still alive
- MaxAge
- 10x Hello timer by default (20 seconds)
- The time the switch will wait before it realizes something is wrong
- Forward Delay
- 15 seconds
- The time between the Listening and Learning state

[dakshinahettiarachchi]

I have never ever seen this kind of clear explanations... this is awesome... keep it up...

[kenchiken]

This will forever be the best STP breakdown out there.

[TheZuuKeeperz]

It's 0549 and its usually really quiet when I study this early in the morning, and the "Arrogant ****" came out of nowhere!! Educational and HILARIOUS, now I have to subscribe!

[siucbset]

Time: Your video was longer than other Spanning Tree informative videos but your video has more information. My time watching your video was time well spent.

[Gearshock]

This is definitely the best explanation of STP I've ever watched. I feel like I actually know something more now instead of being more lost.

[spicyweasel]

I was let down by my local college by not having teachers to teach networking. I had to do the learning all by myself to get my networking specialisation. Your channel is like treasure to me. Such complex topics explained really beautifully. Thanks guys for making this free!

[lowtech]

This is the best and most straightforward breakdown of this topic I've ever seen. I was struggling even with the excellent Wendell book. I truly hope you continue making more videos like these! It's invaluable

[Riad-Eleid]

Thank you, you condensed my doctor's 2 hour lecture into twenty minutes. You explained the topic very well; I enjoyed the mild humor, this video is great.

[qdroghei]

You clarified this subject for me after I spent almost a week banging my head against the wall. Thank you so much. Thank you for the humorous *bleep* with the arrogant router comment. Made my day.

[nikhilkgoel]

My God! I cannot believe I actually understood STP. I do not have a background in networking and have always struggled with understanding network concepts. I am very grateful to you for making these videos. Thank you and keep them coming!

[alexbaker]

This is one of the best Network Tech vids I've ever seen. Not only was each concept clearly explained, but the visual aid for each step of the STP process was outstanding and clear as well.

[blitzip]

Hi, I've recently bought your course for Ccna, 30% through so far and I adore it. The lab work has definitely helped me define the knowledge of the lessons and keep the skills in ny head through trial and error.

[yiannisserpico]

Amazing analysis!! Such a high level networking subject, that too many people mess up, is analysed here in such a way that, even a 10 years old child would do understand. I wish you were my teacher, and I was present in your teaching classrooms. Education needs teachers like you. Thank you!

[jpobi]

I'm amazed by how clearly you've explained it. Also you made me chuckle a couple of times. Props for that. Thank you very much for making this video!!

[mraburles]

the best explanation of STP on the internet

[mameboucarndiaye]

your explanations are like the computer in the film Matrix that people connect behind their heads.
After watching the video you become an expert in the stuff.

[nixtc]

"Now you may not notice, the switches are arrogant * * * *'s, they all think they should be the root bridge" They bloody well are you legend! Watched this 2 times and it all clicked right into place, also because of this legendary quote i will probably not forget this! Thx so very much!

[Allie]

This is an extremely well put together presentation on STP. This will definitely help me to get many of those STP questions on the CCNA exam correct. Thank you for sharing! You rock!

[nicklaw]

Very clear step by step explanation, caught by surprise of your sense of humor. Keep it up :)