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08 - Day 20 - STP Part 1.md

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Day 20 - STP Part 1

Reviewed: No

Redundancy in Networks

Definition and Importance

  • Redundancy: The practice of adding extra components to a network to prevent a single point of failure.
  • Goal: Ensure network availability and minimize downtime, critical for businesses like e-commerce platforms (e.g., Amazon).
  • Implementation: Redundancy should be built into all aspects of network design, including devices, connections, and paths.

Example: Network Design

  1. Poor Design: Single points of failure where a single hardware failure can lead to loss of connectivity.
  2. Better Design: Multiple paths and connections to ensure that a single failure does not disrupt network operations.
  3. Considerations for Redundancy: Important devices like servers may have multiple NICs to connect to different switches, whereas typical PCs might have only one NIC.

Spanning Tree Protocol (STP)

Purpose and Functionality

  • Objective: Prevent Layer 2 loops in a network, which can lead to broadcast storms, network congestion, and MAC address flapping.
  • Broadcast Storms: Occur when broadcast frames loop endlessly in the network, consuming bandwidth and causing severe congestion.
  • MAC Address Flapping: Happens when switches repeatedly update their MAC address tables due to frames with the same source MAC address arriving on different interfaces.

STP Basics

  1. IEEE Standard: 802.1D
  2. Default Behavior: STP is enabled by default on all switches to prevent Layer 2 loops.
  3. Port States:
    • Forwarding: Ports send and receive normal traffic.
    • Blocking: Ports do not send or receive normal traffic but still listen for STP messages (BPDUs).

Key STP Concepts

Bridge and BPDUs

  • Bridge: In STP terminology, a bridge refers to a switch. The term is historical and stems from early networking equipment.
  • BPDU (Bridge Protocol Data Unit): Special frames exchanged by switches to share information and maintain a loop-free network topology.

STP Operation

  1. Root Bridge Election:
    • Bridge ID: Combination of bridge priority and MAC address. The switch with the lowest Bridge ID becomes the root bridge.
    • Bridge Priority: Default value is 32768. Can be adjusted in increments of 4096.
    • MAC Address: Used as a tie-breaker if the bridge priorities are the same.
  2. Bridge ID Structure:
    • Original Structure: 16-bit priority field + 48-bit MAC address.
    • Updated Structure: 4-bit bridge priority + 12-bit VLAN ID (extended system ID) + 48-bit MAC address.
    • PVST (Per-VLAN Spanning Tree): Runs separate STP instances for each VLAN, using VLAN ID in the bridge ID.
  3. Port Roles:
    • Root Ports: The single port on a non-root bridge with the lowest path cost to the root bridge. Forwarding state.
    • Designated Ports: One designated port per collision domain, forwarding state.
    • Non-Designated Ports: Blocking state to prevent loops.
  4. Root Port Selection:
    • Root Cost: Calculated as the sum of the path costs from a switch to the root bridge. Lower cost paths are preferred.
    • Tie-Breakers: If multiple ports have the same root cost, the switch will select the port connected to the neighbor with the lowest Bridge ID. Further ties are broken using port IDs.

STP Path Cost Calculation

  • Path Cost Values:
    • Ethernet (10 Mbps): Cost 100
    • Fast Ethernet (100 Mbps): Cost 19
    • Gigabit Ethernet (1 Gbps): Cost 4
    • 10 Gigabit Ethernet: Cost 2
  • Calculation: The path cost is the sum of the costs on the path to the root bridge.

Port ID

  • Structure: 16-bit value composed of an 8-bit priority and an 8-bit port number.
  • Default Priority: 128
  • Port Number: Unique identifier for each port on a switch, used as the final tie-breaker in root port selection.

Practical Application and Examples

Practice Scenario: Root Bridge and Port Roles

  • Determine Root Bridge: The switch with the lowest Bridge ID is selected.
  • Identify Port Roles: Determine root ports, designated ports, and non-designated (blocking) ports based on STP rules and tie-breakers.

Example Network

  1. Root Bridge Selection: SW3 has the lowest priority or lowest MAC address if priorities are the same.
  2. Root Port Selection: Based on the lowest root cost and tie-breakers such as Bridge ID and Port ID.
  3. Designated Ports: The switch with the lowest root cost to a collision domain will have its port designated. The other port in the collision domain becomes non-designated (blocking).

Spanning Tree Protocol Process Summary

  1. Root Bridge Election: Switch with the lowest Bridge ID is elected as the root bridge.
  2. Root Port Selection: Each non-root switch selects one root port, the port with the lowest root cost to the root bridge.
  3. Designated Port Selection: Each collision domain selects one designated port.
  4. Blocking Ports: Remaining ports are placed in a blocking state to prevent loops.

Commands Used in Day 20

  • Show Spanning Tree Information: show spanning-tree
  • Configure Bridge Priority: spanning-tree vlan [VLAN_ID] priority [PRIORITY]
  • Enable/Disable PortFast: spanning-tree portfast default
  • Show Interface Details: show interface