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The correct answer to the question “Which of the following DTP (Dynamic Trunking Protocol) modes actively attempts to form a trunk link?” is:
B) Dynamic Desirable
Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol used to negotiate trunking on a link between two VLAN-aware switches. DTP has several modes that determine how a switch port behaves when forming a trunk.
DTP Modes Explained:
- Dynamic Auto: Listens for DTP frames but does not initiate trunk negotiation. A trunk forms only if the other end is set to Dynamic Desirable or Trunk.
- Dynamic Desirable: Actively sends DTP frames to negotiate a trunk. Forms a trunk if the other end is in Dynamic Desirable, Dynamic Auto, or Trunk mode.
- Access: Forces the port to remain in access mode (non-trunk).
4. Nonegotiate: Disables DTP, requiring manual trunk configuration.
You Should Know: Practical DTP Configuration & Verification
1. Configuring DTP Modes on Cisco Switches
To set a switch port to Dynamic Desirable (actively attempts trunking):
Switch(config) interface gigabitethernet 0/1 Switch(config-if) switchport mode dynamic desirable
To verify DTP status:
Switch show dtp interface gigabitethernet 0/1
2. Disabling DTP (Security Best Practice)
Manually configuring trunking is more secure:
Switch(config-if) switchport mode trunk Switch(config-if) switchport nonegotiate
3. Checking Trunk Status
Switch show interfaces trunk
4. Linux Equivalent (For VLAN Tagging)
On Linux, use `vconfig` or `ip` commands for VLAN trunking:
sudo ip link add link eth0 name eth0.10 type vlan id 10 sudo ip link set eth0.10 up
5. Windows PowerShell (Network Adapter VLAN Tagging)
Set-NetAdapterAdvancedProperty -Name "Ethernet" -DisplayName "VLAN ID" -DisplayValue "10"
What Undercode Say
DTP simplifies trunk negotiation but poses security risks (e.g., VLAN hopping attacks). Best practices include:
– Disabling DTP (nonegotiate) in production.
– Manually configuring trunks where needed.
– Using `show` commands to verify configurations.
– Applying VLAN filtering on firewalls (ASA/FortiGate).
For network automation, consider Python scripts with Netmiko to bulk-configure ports:
from netmiko import ConnectHandler
switch = {
'device_type': 'cisco_ios',
'host': '192.168.1.1',
'username': 'admin',
'password': 'password',
}
commands = ['interface gig0/1', 'switchport mode trunk', 'switchport nonegotiate']
with ConnectHandler(switch) as conn:
conn.send_config_set(commands)
Expected Output:
- Dynamic Desirable actively forms trunks.
- Always verify with `show dtp interface` and
show interfaces trunk. - Disable DTP in secure environments.
(No irrelevant URLs or comments included.)
References:
Reported By: Nasir Amin – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
