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Difficulty: Very hardInterswitch Connectivity and Trunking (802.1Q)

An administrator is configuring an interswitch trunk link between Switch-A and Switch-B using GigabitEthernet0/1 on both switches. The current running configurations are as follows:

Switch-A:
text
interface GigabitEthernet0/1
switchport mode dynamic desirable
switchport trunk native vlan 20
switchport trunk allowed vlan 10,20,30

Switch-B:
text
interface GigabitEthernet0/1
switchport mode dynamic auto
switchport trunk native vlan 10
switchport trunk allowed vlan 10,20,30

Based on these configuration state parameters, which operational behavior will occur on this interswitch connection?

  1. The link successfully negotiates 802.1Q trunking, but CDP reports a native VLAN mismatch and untagged frame traffic from Switch-A VLAN 20 is processed on Switch-B in VLAN 10.Answer
  2. B
    DTP negotiation fails to establish a trunking operational state because CDP detects mismatched Native VLAN TLV parameters and forces both interfaces into access mode.
  3. C
    The trunk link operates normally without frame leakage because 802.1Q automatically tags native VLAN frames with their explicit VLAN ID whenever a mismatch is detected by Spanning Tree Protocol.
  4. D
    STP immediately transition GigabitEthernet0/1 into a blocking state because PortFast on switch trunk interconnects shuts down links with inconsistent native VLAN settings.

Answer

The link successfully negotiates 802.1Q trunking, but CDP reports a native VLAN mismatch and untagged frame traffic from Switch-A VLAN 20 is processed on Switch-B in VLAN 10.
The combination of 'dynamic desirable' on Switch-A and 'dynamic auto' on Switch-B allows DTP to successfully form an 802.1Q trunk link. Because Switch-A has configured native VLAN 20 and Switch-B has configured native VLAN 10, untagged traffic transmitted by Switch-A for VLAN 20 is interpreted as VLAN 10 by Switch-B upon arrival. CDP routinely verifies native VLAN settings via TLVs and generates continuous native VLAN mismatch alert messages in the console system log.

Step-by-Step Solution

1
Analyze Dynamic Trunking Protocol (DTP) negotiation mode compatibility.
Switch-A uses 'dynamic desirable' (actively attempts to negotiate a trunk) and Switch-B uses 'dynamic auto' (responds to trunk negotiation requests). The operational state of the link becomes an 802.1Q trunk.
Matching dynamic desirable with dynamic auto results in successful trunk establishment.
2
Evaluate native VLAN tagging behavior and traffic path across the trunk.
Switch-A sends VLAN 20 frames untagged over the 802.1Q trunk. Switch-B receives these untagged frames and assigns them to its configured native VLAN, which is VLAN 10.
802.1Q native VLAN traffic is transmitted without a VLAN header tag. The receiving switch places all untagged traffic into its configured native VLAN.
3
Determine Layer 2 discovery protocol and management alert impacts.
Cisco Discovery Protocol (CDP) exchanges neighbor capabilities and detects configured native VLAN 20 on Switch-A versus native VLAN 10 on Switch-B, triggering `%CDP-4-NATIVE_VLAN_MISMATCH` console error messages.
CDP carries native VLAN TLVs to detect trunk configuration inconsistencies across directly connected neighbors.

Key Concept

802.1Q Interswitch Trunking Negotiation and Native VLAN Mismatch Dynamics
Estimated Time:2m 30s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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