802.1D Spanning Tree election rules

[FAQ_User]

Article ID: 4734

Protocols/Features
Spanning Tree

Standards
802.1D
802.1W

Solution
The purpose of 802.1D Spanning Tree is to settle into an efficient, stable configuration in which there are no redundant Layer 2 paths throughout the network. Any redundant links are put into a blocking state.

802.1W (Rapid STP) and 802.1S (Multiple STP) Spanning Tree has the same overall purpose, with some design differences aimed at lower overall traffic disruption when a respan occurs. It is not the goal of this high-level document to explain those differences in detail.

These algorithms use the following set of election rules. For each (numbered) task, subsequent (lettered) rules are only used in order to break an existing tie. When there is a clear winner, no further rule examination is done for that task.

1. Root Bridge election for the whole network
   There is exactly one Root Bridge for each Spanning Tree instance.
   1. Local Bridge ID (Bridge Priority + MAC Address) - lowest prevails
   • Root Port election on each bridge
     Each (non-Root) bridge has exactly one Root Port, which represents the best path to the Root Bridge.
     1. Total Path Cost to root - lowest prevails (local Root Port cost added upon receipt of Configuration BPDUs on that port, from the direction of Root Bridge)
     2. Connected Bridge ID - lowest prevails
     3. Connected Port ID (Port Priority + Port#) - lowest prevails
     4. Local Port ID - lowest prevails
   • Designated Bridge election for each LAN segment
     There is exactly one Designated Bridge giving access to each LAN segment.
     1. Total Path Cost to root - lowest prevails (same as in 2.1, above - only the Root Port's root cost is relevant)
     2. Local Bridge ID (Bridge Priority + MAC Address) - lowest prevails
   • Designated Port election on each Designated Bridge
     There is exactly one Designated Port giving access to each LAN segment.
     1. Local Port ID (Port Priority + Port#) - lowest prevails
   • All ports which are neither Root Ports nor Designated Ports are put into a Blocking state, where Configuration BPDUs are not transmitted but are received, and no user traffic passes. These represent redundant links. Port role elections may be recalculated at any time. With 802.1W, blocked ports are each given an Alternate port (Root port stand-in) or Backup port (Designated port stand-in) role, to pre-stage a more rapid failover.
   A "LAN segment" may support anything from a single user, on up to groups of switches or whole networks.
   
   Default Path Cost information may be found in 4723.