Cisco CCNA (200-301)Network AccessMedium
A network engineer is troubleshooting a connectivity issue between two switches, SW1 and SW2, connected via a single Ethernet cable. SW1's Gi0/1 is configured as 'switchport mode dynamic desirable', and SW2's Gi0/1 is configured as 'switchport mode dynamic auto'. What will be the operational status of the link between SW1 and SW2?
- AThe link will operate as an access port.
- BThe link will form a trunk, but only if the native VLANs match.
- CThe link will not form a trunk and will remain in a non-negotiating state.
- DThe link will form a trunk.
Show answer & explanationAnswer & explanation
Correct answer: D. The link will form a trunk.
Dynamic Desirable actively attempts to form a trunk. Dynamic Auto passively waits for the other side to initiate trunking. When a Dynamic Desirable port connects to a Dynamic Auto port, the Dynamic Desirable port will initiate trunking, and the Dynamic Auto port will respond, resulting in a trunk link.
Why the other options are wrong
- A. An access port would only form if both sides were set to 'dynamic auto' or 'access'.
- B. Native VLAN mismatch would cause issues, but the trunk itself would still form first, then log errors.
- C. DTP negotiation will occur and a trunk will form under these conditions.
DTP Negotiation with Desirable and Auto
When a Dynamic Desirable port connects to a Dynamic Auto port, the Desirable port actively initiates trunking, and the Auto port responds, successfully forming a trunk link.
- Dynamic Desirable: Actively attempts to convert the link into a trunk link.
- Dynamic Auto: Makes the port willing to convert the link to a trunk link if the neighboring port is set to trunk or desirable mode.
- Trunking occurs when at least one side is Desirable, or both sides are Trunk.
Memory trick: Desirable Drives, Auto Agrees: A Trunk Together.