Spanning Tree Protocol Study Guide

Evolution of STP Basics Evolution of STP STP (Spanning Tree Protocol) – Eliminates loops at layer 2 topology

STP STP initially converges on a logically loop-free network topology. Elects one root bridge Selects the root port on all nonroot bridges Selects the designated port on each segment STP Steps (cont) Elects One Root Bridge The protocol uses a process to elect a root bridge Only one bridge acts as the root bridge in a given network per VLAN On the root bridge, all ports act as designated ports Switch with lowest priority parameter is elected as root STP Step 2 (cont) Selects the Root Port on All Non-root Bridges The protocol establishes one root port on each nonroot bridge. The root port is the lowest-cost path from the nonroot bridge to the root bridge

Step 3 Selects Designated Port on Each Segment On each segment, STP establishes one designated port on the bridge that has the lowest path cost to the root bridge The switch primarily chooses a designated port as the least-cost path to the root bridge In the event of a tie, the bridge ID acts as the tiebreaker Port Roles The are four roles of ports under STP 1)Root 2)Designated Port 3)Non-designated Port 4)Disabled

STP Port states:

• Blocking
• Listening
• Learning
• FOrwarding
• Disabled

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RSTP Ports States:

• Discarding
• Learing
• Forwarding

The discarding state of RSTP is equivalent to the Blocking and Listening states of STP

RSTP Port Roles

• Root: The port going toward the root bridge
• Designated: There is one on each segment. It receives the frames from the root port.
• Alternate: A port that is blocked but is an alternate path to the root bridge. Would go into forwarding stat if root port goes down.
• Backup: A port that is blocked because it is a redundant link to the root bridge and has the higher port ID. Would go into forwarding stat if root port goes down.
• Disabled: Plays no role in spanning-tree

RSTP relies on the variables of Link type and Edge ports to achieve fast convergence.

The link types are:

• Point-to-point: full-duplex port
• Shared: half duplex port

An edge port is directly connected to an end point and are allowed to transistion directly into forwarding. This is like portfast except that in an edge port receives a BPDU it will automatically become a normal spanning-tree port.