Spanning Tree Protocol Overview

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