Describe spanning-tree concepts
The basic function of STP is to provide a loop free switched network; this is done by creating a topology of all participating STP switches. The best loop free path through the switched network is then determined from this topology information. The initial step taken by each STP is to elect a root switch; the root switch is used as a central point in a switched network to determine the best route through the switched network . Initially , all switches act as if they are the root switch and do this until they receive traffic from another superior switch (as determined by switch priority); this is referred to as a root switch election.
Another thing that must be understood is that multiple root switches can exist in the network depending on what STP mode is being used. By default, on Cisco switching equipment, each VLAN has its own STP instance and a root switch is elected for each VLAN; this mode is called Per VLAN Spanning Tree Plus (PVST +). If implementing RSTP, Rapid PVST + is used.
Once the root switch is elected, each of the ports is given a role depending on its place within the STP topology; the available port roles when using 802.1D spanning tree are shown below:
● Root—The port given this role is the selected best path to reach the root switch
● Designated—The port given this role is selected with the best path to a specific switched segment; there is only one designated port per switched segment.
● Alternate—The port given this role is selected as a backup to the root port; if the root port should have a problem, this port would take over the root port role.
● Backup—The port given this role is selected as a back to the designated port; if the designated port should have a problem this port would take over the designated port role.
Once the best path is calculated and each of the ports has been given a role , all ports with the alternate or backup STP roles will be blocked to prevent loops.