Spanning Tree 802.1 D Root Bridge,Root Port Designated Port Elections

Spanning-Tree 8021d

Spanning-tree uses control configuration messages called bpdus to communicate Diagrammed below.

There are three types of BPDUs:

• Configuration BPDU (CBPDU), used for Spanning Tree computation
• Topology Change Notification (TCN) BPDU, used to announce changes in the network topology
• Topology Change Notification Acknowledgment (TCA)

General Overview

• A bridge sends a BPDU frame using the unique MAC address of the port itself as a source address, and a destination address of the STP multicast address 01:80:C2:00:00:00.
• BPDUS are exchanged every 2 seconds
• Root Bridge Election Lowest Bridge Priority + Mac
• Priorties go in 4096 increments priorty values 0-64440 with default being  32768
• All ports are designated on root bridge
• Non root brige calculate lowest cost port to root brige if equal it is lowest neighbor bridge id lowst port number. The is known as root port
• All other ports connected to switches are either put into designated or blocking.
• There can be only one designated port per lan segment it is decided lowest cost to the root lowest brige id and lowest port number
• Costs are based on bandwidth fast ethernet 19  1gig is 4 10g is 2
• STP calculation is timer based max age 20 seconds (if bpdu missed how long it waits to flush) forward delay is 15 seconds (how long before transitions fro each state) 
• States are listening,learning,forwarding and blocking
• Bpdus are only sent by the root
• STP Timers can only be adjusted on roots
• TCN occurs when interface is placed in forwading or when interface in forwarding or learning goes to blocking
• TCN do not contain exact change info it is just a way of notifying of change. When a switch recieves a TCN it lowers it mac address aging from default of 5 minutes to 15 seconds
• spanning-tree type ieee
• Design spanning-tree pre instalation to ensure that your desired links to not get blocked i.e fast links getting blocked because of root placement. Also considered that switches computate the best path to the root so the root switch should be a central switch to your network.

Good Commands

Sh spanning-tree 'vlan id'

sh spanning-tree summary

spanning-tree vlan (vlan id) hellow time 'timein seconds'

spanning-tree vlan vlan id forward-time 'time in seconds'

spanning-tree vlan vlan id max-age ' '