Implement and troubleshoot spanning-tree
Spanning Tree Protocol (STP) is a layer-2 protocol that runs on bridges and switches. The specification for STP is IEEE 802.1D. The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network.
Before you configure STP, select a switch to be the root of the spanning tree. After you decide on the root switch, set the appropriate variables to designate the switch as the root switch. The only variable that you must set is the “bridge priority ”. If the switch has a bridge priority that is lower than all the other switches, the other switches automatically select the switch as the root switch.
STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state. If the speed/ duplex of the port is changed, spanning tree recalculates the path cost automatically. A change in the path cost can change the spanning tree topology.
show spantree vlan_id — Shows the current state of the spanning tree for this VLAN ID, from the perspective of the switch on which you issue the command.
● show spantree summary —Provides a summary of connected spanning tree ports by VLAN.
● show spantree statistics —Shows spanning tree statistical information.
● show spantree backbonefast — Displays whether the spanning tree BackboneFast Convergence feature is enabled.
● show spantree blockedports —Displays only the blocked ports.
● show spantree portstate —Determines the current spanning tree state of a Token Ring port within a spanning tree.
● show spantree portvlancost — Shows the path cost for the VLANs on a port.
● show spantree uplinkfast —Shows the UplinkFast settings.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1723-1739). . Kindle Edition.