Day 21 STP (Spanning Tree Protocol) Part2.md

Topics we will cover

• STP states/timers
• STP BPDU
• STP optional features
• STP configuration

Spanning Tree Port States

STP Port State	Stable / Transitional
Blocking	Stable
Listening	Transitional
Learning	Transitional
Forwarding	Stable

• Root/Designated ports remain stable in a forwarding state.
• Non- designated ports remain stable in Blocking state.
• In case of new device addition, interface shutdown, hardware failure they may have to change states.
• Listening and Learning are transitional states which are passed through when an interface is activated, or when Blocking port must transition to a Forwarding state.

Blocking State

• Non designated ports are in Blocking State.
• Interfaces in a Blocking state are effectively disabled to prevent loops.
• Interfaces in a Blocking state do not send/receive regular network traffic. (if arrived, will be dropped)
• Interfaces in this state do not learn MAC addresses. (if regular traffic arrives, it will be dropped without adding mac address to the mac address table)
• Interfaces in this state will receive STP BPDUs (to be aware to Spanning tree topology and be ready to transition towards forward state if needed.)
• But interfaces in this state do not forward STP BPDUs.

Listening State

• After the blocking state, interfaces with Designated or Root role enter Listening State.
  • Only Designated or Root ports enter the Listening state (Non designated ports are always Blocking)
• This state is 15 sec long by default. This is determined by the forward delay timer.
• Interface in this state only forwards/receives STP BPDUs.
• It does not send or receive regular traffic.
• Interfaces in this state do not learn MAC addresses. (if regular traffic arrives, it will be dropped without adding mac address to the mac address table)

Learning State

• After the listening state, a Designated or Root port will enter the Learning state
• This state is 15 sec long by default. This is determined by the forward delay timer.
• Interface in this state only forwards/receives STP BPDUs.
• It does not send or receive regular traffic.
• BUT Interfaces in this state do not learn MAC addresses. (if regular traffic arrives, it will be dropped without adding mac address to the mac address table)

Forwarding State

• Root and Designated ports are in Forwarding State.
• A port in this state operated as normal
  • A port in this state send and receives BPDUs
  • It also sends and receives normal traffic.
  • It also learns mac address from the frames that arrives and add them to the Mac address table

Summary

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Spanning Tree Timers

Hello STP Timer

Forward Delay Timer

Max Age Timer

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Optional Features

Portfast

• Can be enabled in an interfaces connected to end hosts.

BPDU Guard

Other Features

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Configuration

Configure the Primary Root Bridge

Configure the Secondary Root Bridge