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Spanning Tree Protocol (STP)

Definition

Spanning Tree Protocol (STP) is a Layer 2 protocol used by switches to prevent network loops while allowing redundant links for reliability.

STP creates a loop-free logical topology by blocking unnecessary switch ports and automatically enabling them when an active link fails.

ARP (Address Resolution Protocol)

Definition

ARP (Address Resolution Protocol) is used to find the MAC address of a device when its IP address is known.

ARP works between Layer 2 (MAC address) and Layer 3 (IP address).

Key Points

  • ARP maps:
IP Address → MAC Address

  • ARP Request:

    • Sent as a broadcast.
    • Asks: “Who owns this IP address?”

  • ARP Reply:

    • Sent as a unicast.
    • Contains the MAC address of the requested device.

  • ARP table stores learned IP-to-MAC mappings.

Example

Command:

arp -a

Displays the ARP table of a device.

Example output:

192.168.1.10    00-1A-2B-3C-4D-5E

Meaning:

  • IP Address: 192.168.1.10
  • MAC Address: 00-1A-2B-3C-4D-5E

Switch Functions

Definition

A switch performs several important functions to deliver frames efficiently in a network.

The main switch functions are:

  1. Address Learning
  2. Forward Decision
  3. Loop Avoidance

1. Address Learning

Definition

Address learning is the process where a switch learns the source MAC address of incoming frames and stores it in the MAC Address Table.

Process

When a switch receives a frame, it records:

  • Source MAC Address
  • Incoming Port Number
  • VLAN ID

Example:

MAC AddressPortVLAN
AA:BB:CC:DD:EE:01Fa0/11

2. Forward Decision

Definition

A switch checks the destination MAC address in its MAC Address Table to decide where to send a frame.

Cases

Destination MAC Exists

The switch sends the frame only to the specific port.

Example:

PC A → Switch → PC B

The switch forwards the frame directly to PC B.

Destination MAC Does Not Exist

The switch floods the frame.

Flooding means:

  • Sending the frame out of all ports.
  • Except the port where the frame was received.

MAC Address Table

Definition

The MAC Address Table stores learned MAC addresses and their associated switch ports.

Key Points

  • Default aging time:
300 seconds
  • After the aging time expires, unused MAC entries are removed.

Commands

View MAC Address Table:

show mac address-table

View aging time:

show mac address-table aging-time

View static entries:

show mac address-table static

View number of MAC entries:

show mac address-table count

View MAC addresses on an interface:

show mac address-table interface ethernet 0/0

View dynamic entries:

show mac address-table dynamic

Clear MAC Address Table:

clear mac address-table

3. Loop Avoidance

Definition

Switches use STP (Spanning Tree Protocol) to prevent Layer 2 loops when redundant links exist.

Problem: Layer 2 Loop

A loop occurs when multiple paths exist between switches.

Without STP:

  • Broadcast frames continue circulating.
  • Network traffic increases.
  • Broadcast storms may occur.
  • MAC tables become unstable.

Spanning Tree Protocol (STP)

Definition

STP is a Layer 2 protocol that prevents switching loops by blocking redundant ports while keeping backup links available.

Key Points

  • Standard:
IEEE 802.1D
  • Works at:
Layer 2 (Data Link Layer)
  • Enabled by default on Cisco switches.
  • Allows redundant links.
  • Blocks unnecessary ports.
  • Automatically activates blocked ports when failures occur.

STP Operation

STP performs these steps:

  1. Elects a Root Bridge.
  2. Selects Root Ports.
  3. Selects Designated Ports.
  4. Blocks remaining ports.

STP Terminology

Root Bridge

Definition

The Root Bridge is the central switch in the STP topology.

It makes decisions about which ports should forward and which ports should block.

Root Bridge Election

The switch with the lowest Bridge ID (BID) becomes the Root Bridge.

Bridge ID Contains

Bridge ID consists of:

  1. Bridge Priority
  2. System ID Extension
  3. MAC Address

Bridge Priority

Default value:

32768

Range:

0 - 61440

Increment:

4096

Configure Switch Priority

SW(config)# spanning-tree vlan 1 priority 4096

The priority must be a multiple of 4096.

Tie Breaking

If multiple switches have the same priority:

  1. Lowest MAC address wins.

Root Port

Definition

A Root Port is the port on a non-root switch that has the lowest-cost path toward the Root Bridge.

Root Port Selection

Selection order:

  1. Lowest Path Cost
  2. Lowest Upstream Bridge ID
  3. Lowest Port ID

STP Port Costs

Link SpeedCost
Ethernet (10 Mbps)100
Fast Ethernet (100 Mbps)19
Gigabit Ethernet (1000 Mbps)4
10 Gigabit Ethernet2

Designated Port

Definition

A Designated Port is a forwarding port that connects a switch to downstream devices.

Characteristics

  • Always in forwarding state.
  • Selected based on:
  1. Lowest Cost
  2. Lowest Bridge ID
  3. Lowest Port ID

Non-Designated / Alternate Port

Definition

Ports that are placed into blocking state to prevent Layer 2 loops.

Characteristics

  • Located on non-root switches.
  • Receives BPDU frames.
  • Does not forward normal traffic.
  • Becomes active if the forwarding path fails.

BPDU (Bridge Protocol Data Unit)

Definition

BPDU is a control frame exchanged between switches to exchange STP information.

Key Points

Switches use BPDUs to advertise:

  • Bridge priority
  • MAC address
  • STP information

Default transmission:

Every 2 seconds

STP Port States

STP uses different port states during operation.

1. Disabled

Definition

The port is administratively disabled or not operational.

Characteristics:

  • Does not send BPDU.
  • Does not forward traffic.

2. Blocking

Definition

The port prevents loops by blocking normal data traffic.

Characteristics:

  • Receives BPDU.
  • Does not send user data.
  • Can become forwarding if another path fails.

3. Listening

Definition

The port participates in STP calculations.

Characteristics:

  • Sends and receives BPDU.
  • Does not send user data.

4. Learning

Definition

The switch begins learning MAC addresses.

Characteristics:

  • Sends and receives BPDU.
  • Learns MAC addresses.
  • Does not forward user traffic yet.

5. Forwarding

Definition

The port is fully operational.

Characteristics:

  • Sends and receives BPDU.
  • Learns MAC addresses.
  • Sends and receives data frames.

Cisco Switch Management Notes

Switch Boot Process

The boot process of a switch is similar to a router.

Steps:

  1. POST executes
  2. IOS loads from Flash Memory into RAM
  3. Startup configuration loads from NVRAM into RAM

Switch Access Methods

A switch can be accessed using:

  • Console
  • Telnet
  • SSH

Cisco Switch Modes

User EXEC Mode

Prompt:

Switch>

Used for basic monitoring.

Privileged EXEC Mode

Prompt:

Switch#

Used for viewing and changing configurations.

Global Configuration Mode

Prompt:

Switch(config)#

Used for device configuration.

STP Verification Commands

Show STP Information

show spanning-tree

Displays complete STP information.

Show Root Bridge

show spanning-tree root

Displays Root Bridge information.

Show Interface STP Details

show spanning-tree interface ethernet 0/0 detail

Displays detailed STP information for an interface.

Common Mistakes

  • Confusing ARP with DNS:

    • ARP resolves IP addresses to MAC addresses.
    • DNS resolves domain names to IP addresses.

  • Forgetting that ARP Reply is unicast.

  • Assuming switches automatically prevent loops without STP.

  • Confusing Root Port and Designated Port.

  • Choosing the highest Bridge ID as Root Bridge; STP chooses the lowest Bridge ID.

  • Forgetting that blocked ports still receive BPDU frames.

  • Assuming STP removes redundant links permanently; it only blocks them temporarily.

Short Exam Notes

  • ARP: Finds MAC address using an IP address.

  • Switch Learning: Learns source MAC addresses and stores them in the MAC table.

  • STP: Prevents Layer 2 loops.

  • STP Standard: IEEE 802.1D.

  • Root Bridge: Switch with the lowest Bridge ID.

  • Bridge ID: Priority + System ID Extension + MAC Address.

  • Default Bridge Priority: 32768.

  • Root Port: Lowest-cost path toward Root Bridge.

  • Designated Port: Forwarding port toward downstream devices.

  • Alternate Port: Blocking port used to prevent loops.

  • BPDU: STP control message exchanged every 2 seconds.

  • Port States:

    • Disabled
    • Blocking
    • Listening
    • Learning
    • Forwarding

  • Verification:

    • show spanning-tree
    • show spanning-tree root