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06 - Day 18 - VLANs Part 3.md

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Day 18 - VLANs Part 3

Reviewed: No

Dynamic Trunking Protocol (DTP)

  • Purpose: Automatically negotiates switchport status (access or trunk) between Cisco switches.
  • Enabled by Default: On all Cisco switch interfaces.
  • Modes:
    • Dynamic Desirable: Actively tries to form a trunk with another switch.
    • Dynamic Auto: Passive mode; forms a trunk if the other end is actively trying.

Key Concepts and Commands

  1. DTP Modes Explained:
    • Dynamic Desirable: Forms a trunk if the connected port is in trunk, dynamic desirable, or dynamic auto mode.
    • Dynamic Auto: Forms a trunk if the connected port is in trunk or dynamic desirable mode.
    • Manual Configuration: Recommended to disable DTP for security purposes.
    • switchport mode dynamic: Command to configure DTP mode.
    • show interfaces switchport: Displays administrative and operational mode details.
  2. Scenarios and Outputs:
    • Dynamic Desirable + Trunk: Forms a trunk.
    • Dynamic Auto + Trunk: Forms a trunk.
    • Dynamic Auto + Dynamic Auto: Operates as access ports.
    • Trunk + Access: Misconfiguration; should not occur in a real network.
  3. DTP Security Considerations:
    • Disabling DTP: Recommended on all interfaces to prevent potential exploits.
    • switchport nonegotiate: Stops sending DTP frames, effectively disabling DTP on the interface.
  4. Trunk Encapsulation:
    • Dot1q vs. ISL: Trunk encapsulation can be negotiated, with ISL favored if both support it.
    • DTP Frames: Sent in VLAN1 (ISL) or native VLAN (dot1q).

VLAN Trunking Protocol (VTP)

  • Purpose: Centralizes VLAN management, synchronizes VLAN databases across multiple switches.
  • Modes:
    • Server: Default mode; can add, modify, and delete VLANs.
    • Client: Cannot modify VLANs; syncs database from server.
    • Transparent: Does not participate in VTP domain synchronization; forwards VTP advertisements.

Key Concepts and Commands

  1. VTP Modes Explained:
    • Server Mode: Stores VLAN database in NVRAM, advertises VLANs.
    • Client Mode: Syncs VLAN database from server, does not store in NVRAM (except VTPv3).
    • Transparent Mode: Maintains independent VLAN database, forwards VTP advertisements without syncing.
  2. VTP Configuration:
    • VTP Domain: Must be the same across switches to sync VLANs.
    • VTP Version: Versions 1, 2, and 3; version 3 supports extended VLANs (1006-4094).
    • show vtp status: Displays VTP domain name, mode, version, revision number, etc.
  3. VTP Synchronization and Risks:
    • Revision Number: Key factor in synchronization; the highest number indicates the most up-to-date VLAN database.
    • Risk of Old Switches: Connecting an old switch with a higher revision number can overwrite the network's VLAN database, causing loss of connectivity.
  4. VTP Management:
    • Resetting Revision Number: Change the domain name or set the switch to transparent mode.
    • Danger of VTP: Caution needed when adding new switches to prevent unintentional network disruption.

Review and Summary

  • DTP: Enables dynamic negotiation of trunk/access ports; should be disabled for security.
  • VTP: Centralized VLAN management; rarely used due to potential risks.
  • Commands Overview:
    • DTP Commands: switchport mode dynamic, switchport nonegotiate, show interfaces switchport
    • VTP Commands: vtp mode, vtp domain, show vtp status

Commands Used in Day 19

  • DTP:
    • switchport mode dynamic desirable
    • switchport mode dynamic auto
    • switchport mode access
    • switchport mode trunk
    • switchport nonegotiate
    • show interfaces switchport
  • VTP:
    • vtp mode server
    • vtp mode client
    • vtp mode transparent
    • vtp domain [name]
    • show vtp status
    • show vlan brief