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Difficulty: HardInterswitch Connectivity and Trunking (802.1Q)

Two Cisco Catalyst switches, SwitchA and SwitchB, are connected via their GigabitEthernet0/1 interfaces. SwitchA is configured with the commands `switchport mode dynamic desirable` and `switchport trunk native vlan 20`. SwitchB is configured with `switchport mode dynamic auto` and default interface settings (`switchport trunk native vlan 1`). Based on this operational configuration, which statement correctly describes the trunking state and frame handling across this link?

  1. The link successfully negotiates an 802.1Q trunk, but untagged frames sent from SwitchA's native VLAN 20 will be processed in VLAN 1 on SwitchB, causing a cross-VLAN traffic leak and triggering CDP error logs.Answer
  2. B
    The link fails to negotiate an 802.1Q trunk because both operational ends must have matching native VLAN configurations before Dynamic Trunking Protocol (DTP) can establish a trunking operational state.
  3. C
    The trunk link forms successfully, but SwitchA automatically tags frames from VLAN 20 with an 802.1Q header containing VLAN ID 1 to force alignment with SwitchB's native VLAN setting.
  4. D
    The link forms a trunk, but Cisco Discovery Protocol (CDP) immediately forces both GigabitEthernet0/1 interfaces into the err-disabled state upon detecting the native VLAN TLV mismatch.

Answer

The link successfully negotiates an 802.1Q trunk, but untagged frames sent from SwitchA's native VLAN 20 will be processed in VLAN 1 on SwitchB, causing a cross-VLAN traffic leak and triggering CDP error logs.
The combination of dynamic desirable on one side and dynamic auto on the other successfully forms an 802.1Q trunk via Dynamic Trunking Protocol. Because native VLAN traffic is sent untagged by default under 802.1Q, frames originating in VLAN 20 on the first switch are received untagged on the second switch and mapped to its native VLAN 1. This creates a cross-VLAN leak and causes Cisco Discovery Protocol to log warning messages regarding the native VLAN mismatch.

Step-by-Step Solution

1
Evaluate DTP negotiation between SwitchA and SwitchB.
SwitchA is set to dynamic desirable (actively seeking to form a trunk) and SwitchB is set to dynamic auto (willing to form a trunk if requested). This combination successfully negotiates an operational trunk link.
DTP operational mode matrix dictates that desirable + auto results in a trunking state.
2
Analyze 802.1Q native VLAN tagging mechanics across the trunk.
SwitchA sends VLAN 20 traffic without an 802.1Q tag. When SwitchB receives untagged traffic on a trunk port, it forwards that traffic to its own configured native VLAN (VLAN 1).
By default, 802.1Q trunks transmit native VLAN frames without inserting a 4-byte VLAN tag header.
3
Determine control plane diagnostic behavior.
CDP exchanges native VLAN information via TLVs and generates console log warnings about the native VLAN mismatch, but traffic forwarding continues and the interface remains up/up.
CDP provides notification of misconfigurations but does not automatically shut down interfaces or block native VLAN traffic unless explicitly combined with STP loop guard/BPDU guard mechanisms.

Key Concept

802.1Q Native VLAN Tagging Mechanics and DTP Negotiation
Estimated Time:2m 0s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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