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Cisco DTP and VTP Overview

Jul 27, 2025

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Overview

This lecture covers Cisco’s proprietary DTP (Dynamic Trunking Protocol) and VTP (VLAN Trunking Protocol), their operational modes, security considerations, and relevant configurations for the CCNA exam.

DTP: Dynamic Trunking Protocol

  • DTP is a Cisco protocol that negotiates switchport status as access or trunk without manual commands.
  • DTP is enabled by default on Cisco switches; applies only to Cisco devices.
  • DTP modes: dynamic auto (passive, forms trunk if neighbor requests) and dynamic desirable (actively tries to form trunk).
  • Manual configuration with switchport mode access or switchport mode trunk is recommended for security.
  • DTP can be disabled with switchport nonegotiate or by using access mode.
  • Trunk links may use ISL or dot1q encapsulation, negotiated by DTP if supported.
  • Default DTP mode: dynamic desirable on older switches, dynamic auto on newer switches.
  • DTP forms trunks only with other switches—not routers or PCs.
  • Trunking operational outcomes depend on mode combinations (e.g., two dynamic autos = access, dynamic auto + desirable = trunk).

VTP: VLAN Trunking Protocol

  • VTP allows central VLAN configuration on a server switch and synchronizes VLAN databases across switches in the same VTP domain.
  • Three VTP modes: server (default, can add/edit VLANs, syncs to highest revision number), client (cannot add/edit VLANs, syncs to server), transparent (maintains independent VLAN database, forwards but does not act on VTP advertisements).
  • VTP advertisements are sent only on trunk ports.
  • All switches in a VTP domain must have the same domain name to synchronize.
  • The VTP configuration revision number is incremented with every VLAN change and used to determine which VLAN database is current.
  • Major risk: connecting a switch with a higher revision number can overwrite the VLAN database on all switches in the domain.
  • Revision number resets to 0 when changing VTP domain or switching to transparent mode.
  • VTP version 1 and 2 support VLANs up to 1005; version 3 supports extended VLANs (1006–4094).
  • VTP is not recommended for use in modern networks due to potential dangers.

Quiz Q&A Summary

  • Old switches default to dynamic desirable mode, new ones to dynamic auto.
  • Use vtp mode transparent to forward VTP advertisements without syncing VLANs.
  • Reset VTP revision number by changing domain or switching to transparent mode.
  • DTP trunking operational modes depend on the combination of local and neighbor interface modes.

Key Terms & Definitions

  • DTP (Dynamic Trunking Protocol) — Cisco protocol that negotiates automatic trunk or access port configuration between switches.
  • VTP (VLAN Trunking Protocol) — Cisco protocol for synchronizing VLAN databases across multiple switches in a VTP domain.
  • Trunk Port — Switchport carrying multiple VLANs between switches.
  • Access Port — Switchport assigned to a single VLAN.
  • VTP Domain — Group of switches sharing VLAN information via VTP with the same domain name.
  • Configuration Revision Number — Number used in VTP to indicate the most recent VLAN database version.

Action Items / Next Steps

  • Practice DTP and VTP configurations in Packet Tracer.
  • Review DTP and VTP operational mode tables.
  • Download and study the Anki flashcard deck provided.
  • Complete the accompanying Packet Tracer lab in the next video.
  • Review today’s quiz questions and explanations.

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