VLANCCNA 200-301

DTP Auto Mode Preventing Trunk from Forming

Presenting Symptom

A trunk link between two switches fails to form, and the interface remains in access mode despite configuring switchport mode trunk on one side.

Network Context

In a small branch office, two Cisco Catalyst 2960 switches running IOS 15.0 are connected via a crossover cable on GigabitEthernet0/1. The network administrator configured one switch with switchport mode trunk, but the other switch is left in the default Dynamic Auto mode. The trunk does not form, and VLAN traffic is not passing between the switches.

Diagnostic Steps

Check interface status and trunking

show interfaces trunk

No trunk interfaces are listed.

If no trunk interfaces appear, the trunk is not operational. This indicates a problem with DTP negotiation or configuration mismatch.

Check interface switchport mode

show interfaces gigabitethernet0/1 switchport

Administrative Mode: trunk
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: On

The administrative mode is trunk, but operational mode is static access. This suggests DTP negotiation failed because the other side is not set to trunk or desirable.

Check the neighbor switch's interface configuration

show running-config interface gigabitethernet0/1

interface GigabitEthernet0/1
 switchport mode dynamic auto

The neighbor is in dynamic auto mode, which will not actively initiate trunking. It will only respond to DTP requests. Since the local side is in trunk mode (which sends DTP frames), the trunk should form. However, if the local side is in trunk mode but DTP is disabled (switchport nonegotiate), the trunk will not form. Check if switchport nonegotiate is configured.

Verify DTP negotiation status

show dtp interface gigabitethernet0/1

DTP information for GigabitEthernet0/1:
TOS/TAS/TNS: TRUNK/TRUNK/TRUNK
TOT/TAT/TNT: 1-5/1-5/1-5
Neighbor 1 MAC address: 0000.0000.0000
Neighbor 2 MAC address: 0000.0000.0000
Neighbor 3 MAC address: 0000.0000.0000

If neighbor MAC addresses are all zeros, DTP is not receiving frames from the neighbor. This could be due to a cable issue, or the neighbor has switchport nonegotiate configured, which disables DTP.

Root Cause

The trunk fails because one switch is configured with switchport mode trunk (which sends DTP frames) but the other switch is in dynamic auto mode (which only responds to DTP frames). However, if the trunk mode switch also has switchport nonegotiate configured, it will not send DTP frames, and the dynamic auto switch will never initiate trunking. In this scenario, the trunk mode switch has switchport nonegotiate, preventing DTP negotiation.

Resolution

Remove the switchport nonegotiate command from the trunk mode switch to allow DTP frames to be sent. Alternatively, configure the other switch with switchport mode trunk or switchport mode dynamic desirable. Commands: 1. On the switch with trunk mode: interface gigabitethernet0/1 no switchport nonegotiate (This re-enables DTP negotiation) 2. Alternatively, on the dynamic auto switch: interface gigabitethernet0/1 switchport mode trunk (This forces trunking without relying on DTP)

Verification

Run 'show interfaces trunk' and verify that the trunk interface appears. Also run 'show interfaces gigabitethernet0/1 switchport' and confirm that Operational Mode shows 'trunk'. Expected output: - show interfaces trunk: lists the trunk with VLANs allowed. - show interfaces gigabitethernet0/1 switchport: Operational Mode: trunk

Prevention

1. Avoid using switchport nonegotiate unless necessary for security; it can cause trunking failures. 2. Use consistent trunking configuration on both ends, such as switchport mode trunk on both sides. 3. Use switchport mode dynamic desirable on both ends to allow DTP to negotiate trunking automatically.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests understanding of DTP modes and trunk negotiation. Questions may present a troubleshooting scenario where a trunk fails to form, and you must identify the DTP mode mismatch or the effect of switchport nonegotiate. The exam expects you to know that dynamic auto will not initiate trunking, and that switchport nonegotiate disables DTP.

Exam Tips

Remember that dynamic auto will not actively send DTP frames; it only responds. Dynamic desirable will actively negotiate.

switchport nonegotiate disables DTP entirely; if used on one side, the other side must be configured with switchport mode trunk to form a trunk.

Know the 'show interfaces trunk' and 'show interfaces switchport' commands to verify trunk status and DTP negotiation.

Commands Used in This Scenario

show interfaces switchport

Displays the administrative and operational status of a switch port, including VLAN membership, trunking mode, and access VLAN, used to verify VLAN configuration and port security settings.

show interfaces trunk

Displays trunk interface status, allowed VLANs, and pruning information for all trunk ports on a Cisco switch, used to verify trunking configuration and VLAN membership.

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