Hands-On Lab: Setting Switchports to Dynamic Trunking Modes

Lab Objective

Configure both dynamic desirable and dynamic auto switchport modes between two switches, observe which combinations successfully negotiate a trunk automatically via DTP, and identify the one combination that does not.

Lab Purpose

Before explicitly configuring switchport mode trunk, as done in earlier labs, engineers should understand how Dynamic Trunking Protocol (DTP) can negotiate this automatically — and, just as importantly, why most production environments disable DTP entirely in favor of explicit configuration for security and predictability.

Lab Topology

Switch1 ---- Gi1/0/24 ---- Gi1/0/24 ---- Switch2

Link currently has no explicit switchport
mode configured on either side

Task 1: Configure Dynamic Desirable on Both Sides

Set Gi1/0/24 on both switches to dynamic desirable, and verify whether a trunk forms.

Task 2: Configure Dynamic Auto on Both Sides

Change both sides to dynamic auto, and verify whether a trunk forms this time.

Task 3: Configure One Side Desirable and the Other Auto

Set Switch1 to dynamic desirable and Switch2 to dynamic auto, and verify the result.

Task 4: Identify the Non-Negotiating Combination

Explain which combination of dynamic modes does NOT result in a trunk forming, based on the pattern observed.

Solution and Verification

Switch1(config)# interface gigabitethernet1/0/24
Switch1(config-if)# switchport mode dynamic desirable

Switch2(config)# interface gigabitethernet1/0/24
Switch2(config-if)# switchport mode dynamic desirable

Switch1# show interfaces gigabitethernet1/0/24 switchport | include Mode

Administrative Mode: dynamic desirable
Operational Mode: trunk
-- Desirable + Desirable successfully negotiates
-- a trunk, since both sides actively offer

Switch1(config)# interface gigabitethernet1/0/24
Switch1(config-if)# switchport mode dynamic auto

Switch2(config)# interface gigabitethernet1/0/24
Switch2(config-if)# switchport mode dynamic auto

Switch1# show interfaces gigabitethernet1/0/24 switchport | include Mode

Administrative Mode: dynamic auto
Operational Mode: static access
-- Auto + Auto does NOT form a trunk -- neither
-- side actively initiates, so both remain
-- passively waiting and default to access mode

Switch1(config)# interface gigabitethernet1/0/24
Switch1(config-if)# switchport mode dynamic desirable

Switch2(config)# interface gigabitethernet1/0/24
Switch2(config-if)# switchport mode dynamic auto

Switch1# show interfaces gigabitethernet1/0/24 switchport | include Mode

Operational Mode: trunk
-- Desirable + Auto DOES form a trunk, since
-- the desirable side actively initiates and
-- the auto side responds when asked

Key Takeaway

DTP negotiation mirrors the PAgP negotiation logic covered in an earlier EtherChannel lab: Desirable+Desirable and Desirable+Auto both form a trunk, while Auto+Auto never does, since neither side takes the initiative. In practice, most security-conscious production networks disable DTP entirely (using switchport nonegotiate alongside explicit trunk or access mode), since relying on automatic negotiation on an access port an attacker could physically reach represents an unnecessary risk.