VTP and Spanning Tree Protocol (STP) for Network Management

Posted on Dec 1, 2024 in Computers

VLAN Trunking Protocol (VTP)

VTP allows a network administrator to propagate VLAN configurations across a network.

Benefits of VTP

  • Consistency in VLAN configuration.
  • Accurate VLAN tracking and monitoring.
  • Dynamic reports on added VLANs.
  • Dynamic trunk configuration.

VTP automatically synchronizes domain and VLAN configurations. Use show vtp status to display network status and recommended actions.

VTP Modes

  • Server: Create, modify, and delete VLANs (default mode).
  • Client: Receives and applies VTP information from servers.
  • Transparent: Forwards VTP advertisements but does not modify its own VLAN database.

VTP Message Format

VTP information is encapsulated within Ethernet frames, consisting of a header and a message field.

Layer 2 Redundancy and Spanning Tree Protocol (STP)

Layer 2 redundancy enhances network availability by providing alternate paths. However, it can lead to Layer 2 loops if not managed.

Disadvantages of Redundancy without STP

  • Layer 2 Loops: Multiple paths between devices can cause loops.
  • Broadcast Storms: Loops amplify broadcast frames, consuming bandwidth.
  • Duplicate Unicast Frames: Loops can cause unicast frames to be duplicated.

STP prevents loops by ensuring only one logical path between network destinations.

Spanning Tree Algorithm (STA)

STA determines which switch ports to block to prevent loops. A designated Root Bridge serves as a reference point for all calculations.

Bridge Protocol Data Units (BPDUs)

BPDUs are hello packets exchanged between bridges to share STP information.

Configuring and Verifying STP

Configure root bridge priority with:

spanning-tree vlan vlan-id root primary

spanning-tree vlan vlan-id root secondary

Verify with: show spanning-tree

STP Port Roles

  • Root Port: Best path to the root bridge on non-root bridges.
  • Designated Port: Forwards traffic away from the root bridge. All ports on the root bridge are designated.
  • Non-Designated Port: A blocked port that prevents loops.
  • Disabled Port: Administratively shut down port.

Verify the root port with the command show spanning-tree in privileged EXEC mode.

Types of STP

  • Per-VLAN Spanning Tree (PVST): Maintains a spanning-tree instance for each VLAN (Cisco proprietary).
  • Per-VLAN Spanning Tree Plus (PVST+): PVST with Cisco proprietary extensions.
  • Rapid PVST+: Faster convergence than STP (based on IEEE 802.1w) with Cisco proprietary extensions.
  • Rapid Spanning Tree Protocol (RSTP): Faster convergence after topology changes (IEEE 802.1w).
  • Multiple Spanning Tree Protocol (MSTP): Maps multiple VLANs to the same spanning-tree instance to reduce overhead.

RSTP (IEEE 802.1w) is an evolution of the 802.1D standard, maintaining similar terminology and parameters for ease of transition.