Spanning Tree Protocol Overview

Study Notes

A redundant network is designed with multiple switches and redundant paths to ensure that if one switch fails, another path is available.
Redundant links can also share the traffic load and increase capacity.
Redundant devices provide an alternate default gateway if the primary gateway is unavailable.
Redundancy can cause a Layer 2 loop, which is when a network has multiple paths to a destination.
Ethernet LANs require a loop-free topology with a single path between each device.
STP is a protocol used to prevent such loops while allowing for redundancy in a network.
IEEE 802.1D is the original standard for STP.

Without STP enabled, Layer 2 loops can form, causing broadcasts and unknown unicast frames to loop endlessly.
This can bring down a network quickly, often within a few seconds.
Unknown unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device.
A Layer 2 loop can result in three primary issues: MAC database instability, broadcast storm, and multiple frame transmission.
STP ensures that there is a single logical path between all destinations in the network by blocking redundant paths.
A port is blocked when user data is prevented from entering or leaving that port.
If needed, STP recalculates paths and unblocks ports to activate a redundant path.

STP uses the Spanning Tree Algorithm (STA) to determine which ports to block.
STP guarantees only a single logical path between destinations by intentionally blocking redundant paths.
Each switch has a single path to the root bridge.
When network cables or switches fail, STP recalculates paths and unblocks necessary ports.
Switches use Bridge Protocol Data Units (BPDUs) to share information about themselves and their connections.
BPDUs are used to elect the root bridge, root ports, designated ports, and alternate ports.
Each BPDU contains a bridge ID (BID), which identifies the sending switch.
The BID contains a priority value, the MAC address of the switch, and an extended system ID.
The lowest BID value is determined by the combination of these three fields.
The default priority value for all switches is 32768.

The default priority value for all switches is 32768.
The lower the priority value, the higher the priority of the switch in the network.
The priority of a switch can be changed to influence the election of the root bridge.
If switches have the same priority, the switch with the lowest MAC address becomes the root bridge.

Each switch has one root port.
This port is the primary path towards the root bridge.
The root port on the root bridge is always in forwarding mode.
The root port of each switch is the one with the lowest total cost to the root bridge.

The alternate port is blocked and prevented from forwarding traffic.
It is a backup path that becomes active if a switch or connection fails.
It is typically used to prevent loops in the network.
It allows the network to remain operational even if a switch or connection fails.
If a switch fails, STP will recalculate the ports and possibly change the blocked ports to designated ports to restore connectivity.