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Difficulty: HardInterswitch Connectivity and Trunking (802.1Q)

An enterprise network switch, Switch-A, is connected to Switch-B via interface GigabitEthernet0/1. Switch-A is configured with `switchport mode dynamic desirable` and `switchport trunk native vlan 50`. Switch-B is configured with `switchport mode dynamic auto` and `switchport trunk native vlan 1`. Spanning Tree Protocol (STP) and Cisco Discovery Protocol (CDP) are enabled on both switches. Which TWO operational outcomes occur on this interswitch link? (Select TWO.)

  1. The interface operational mode successfully negotiates to an IEEE 802.1Q trunk link.Answer
  2. CDP generates periodic console messages warning of a native VLAN mismatch between the switches.Answer
  3. C
    The interface fails DTP negotiation and transitions to an access port in VLAN 1 on both switches.
  4. D
    Frames originating from VLAN 50 on Switch-A are dropped at ingress by Switch-B due to invalid 802.1Q tags.

Answer

The interswitch link successfully negotiates an operational IEEE 802.1Q trunk via DTP, and CDP periodically logs native VLAN mismatch notifications.
An IEEE 802.1Q trunk forms successfully because a port configured as dynamic desirable actively negotiates with a port in dynamic auto mode. Additionally, because Switch-A defines VLAN 50 as native while Switch-B defines VLAN 1 as native, CDP detects the inconsistency through TLV messages and generates native VLAN mismatch warnings.

Step-by-Step Solution

1
Analyze the Dynamic Trunking Protocol (DTP) operational modes configured on both ends of the link.
Switch-A (`dynamic desirable`) actively sends DTP frames, and Switch-B (`dynamic auto`) listens and responds.
The combination of dynamic desirable and dynamic auto results in successful trunk negotiation.
2
Analyze IEEE 802.1Q native VLAN egress tagging and ingress processing behavior.
Switch-A sends VLAN 50 frames untagged, while Switch-B expects native VLAN traffic on VLAN 1 untagged.
By default, 802.1Q trunks transmit frames belonging to the configured native VLAN without an 802.1Q header.
3
Evaluate Layer 2 discovery protocol diagnostics for native VLAN consistency.
CDP exchanges native VLAN information via TLVs, detects the mismatch (VLAN 50 vs VLAN 1), and logs `%CDP-4-NATIVE_VLAN_MISMATCH` console messages.
CDP continuously monitors interswitch link parameters to highlight Layer 2 misconfigurations.

Key Concept

DTP operational mode negotiation and IEEE 802.1Q native VLAN mismatch behavior
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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