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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

An administrator is configuring a trunk link between two Cisco Catalyst switches. Interface GigabitEthernet0/1 on Switch-1 is configured with `switchport mode dynamic auto` and `switchport trunk native vlan 50`. The connected interface GigabitEthernet0/1 on Switch-2 is configured with `switchport mode dynamic desirable` and `switchport trunk native vlan 10`. Which outcome will occur on this interswitch link?

  1. The trunk link forms successfully, but CDP logs periodic native VLAN mismatch error messages, and untagged traffic on VLAN 50 from Switch-1 will leak into VLAN 10 on Switch-2.Answer
  2. B
    Dynamic Trunking Protocol (DTP) negotiation fails completely because matching native VLAN IDs are required prior to establishing a trunking operational state.
  3. C
    The interface is placed into an err-disabled state by CDP as soon as the CDP TLV detects the mismatching native VLAN configuration.
  4. D
    PortFast must be enabled on the trunk interface on Switch-1 to bypass Spanning Tree Protocol forwarding delays and allow native VLAN alignment.

Answer

The interswitch link negotiates successfully into an 802.1Q trunk state because dynamic auto paired with dynamic desirable forms a trunk. However, because the native VLAN numbers do not match across switches, untagged traffic sent by Switch-1 on VLAN 50 will be treated as belonging to VLAN 10 by Switch-2, and CDP will log continuous native VLAN mismatch warnings.
When configuring DTP, pairing dynamic auto with dynamic desirable successfully forms an operational trunk. However, IEEE 802.1Q leaves native VLAN frames untagged. Switch-1 sends VLAN 50 traffic untagged, while Switch-2 interprets all incoming untagged traffic as belonging to its native VLAN (VLAN 10), leading to cross-VLAN traffic leakage and CDP logging native VLAN mismatch warnings.

Step-by-Step Solution

1
Analyze DTP negotiation modes between Switch-1 and Switch-2.
Dynamic auto (passive waiting) combined with dynamic desirable (active requesting) results in an operational trunk link.
DTP modes determine whether the port becomes an access or trunk port regardless of VLAN tagging options.
2
Analyze IEEE 802.1Q native VLAN behavior across the link.
Untagged frames sent from VLAN 50 on Switch-1 cross the link without an 802.1Q header and are ingested into VLAN 10 by Switch-2.
802.1Q does not tag frames belonging to the configured native VLAN of the transmitting switch interface.
3
Evaluate CDP control plane protocol alerts.
CDP exchanges native VLAN information via TLVs and generates console log warnings while leaving the interface in the forwarding state.
CDP identifies configuration mismatches between neighbors but does not automatically err-disable the port.

Key Concept

802.1Q DTP Negotiation Mechanics and Native VLAN Mismatch Consequences
Estimated Time:1m 30s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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