Spanning Tree Protocol

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Spanning Tree Protocol

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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 

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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 

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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 

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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 

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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 

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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 

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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 

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Downfall of STP -- Convergence

\- The time it takes to do the work and become stable 

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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