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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

A network engineer connects interface GigabitEthernet0/1 on Switch-A to interface GigabitEthernet0/1 on Switch-B. Switch-A is configured with `switchport mode trunk` and `switchport trunk native vlan 50`. Switch-B retains default Cisco IOS switchport settings. Which two operational outcomes will occur on this interswitch connection? (Select two.)

  1. Switch-B operational mode transitions to an 802.1Q trunk through Dynamic Trunking Protocol (DTP) negotiation.Answer
  2. CDP generates system log messages reporting a native VLAN mismatch between Switch-A and Switch-B.Answer
  3. C
    The interswitch connection fails to form a trunk because dynamic auto ports require dynamic desirable mode on the remote end to establish trunking.
  4. D
    Spanning Tree Protocol automatically places both switch ports into PortFast forwarding to prevent native VLAN frame drops.

Answer

The interswitch link operational outcomes are: Switch-B operational mode transitions to an 802.1Q trunk through Dynamic Trunking Protocol (DTP) negotiation, and CDP generates system log messages reporting a native VLAN mismatch between Switch-A and Switch-B.
Default Cisco Catalyst interface settings use `switchport mode dynamic auto` and native VLAN 1. When connected to a port configured with `switchport mode trunk`, DTP negotiates an operational 802.1Q trunk link. Concurrently, because Switch-A is explicitly set to native VLAN 50 while Switch-B remains on VLAN 1, CDP exchanges uncover the configuration discrepancy and issue native VLAN mismatch error logs.

Step-by-Step Solution

1
Analyze the Dynamic Trunking Protocol (DTP) operational states.
Switch-A is in static `trunk` mode (which actively sends DTP frames). Switch-B is in default `dynamic auto` mode. The combination of `trunk` and `dynamic auto` results in a negotiated 802.1Q trunk.
Ports configured as `dynamic auto` passively listen for DTP trunking requests and will agree to negotiate trunking if requested by `trunk` or `dynamic desirable` modes.
2
Analyze the Native VLAN configuration on both sides of the trunk.
Switch-A has native VLAN set to 50, whereas Switch-B retains the default native VLAN 1. This creates a native VLAN mismatch.
802.1Q trunks send untagged frames for the configured native VLAN. When native VLAN IDs differ across a link, CDP exchanges TLVs that detect the discrepancy and log native VLAN mismatch warnings.

Key Concept

802.1Q Trunk Negotiation & Native VLAN Mismatch Diagnostics
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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