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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

Two Cisco Catalyst switches, SW1 and SW2, are connected via their GigabitEthernet0/2 interfaces. SW1 is configured with `switchport mode dynamic auto` and allows VLANs 10, 20, and 30 on its trunk. SW2 is configured with `switchport mode dynamic desirable` and has its allowed VLAN list manually pruned using `switchport trunk allowed vlan 10,20`. Both switches share native VLAN 10. Which behavior occurs when a host on VLAN 30 attached to SW1 sends a broadcast frame destined across this switch interconnect?

  1. The trunk link negotiates successfully, but SW2 drops the VLAN 30 broadcast frame because VLAN 30 is absent from SW2's allowed VLAN list.Answer
  2. B
    The trunk link fails to form and falls back to access mode because SW1 and SW2 have inconsistent allowed VLAN lists, triggering a native VLAN mismatch error.
  3. C
    SW2 receives the frame as untagged native VLAN traffic because CDP TLV messages automatically reassign pruned VLANs to native VLAN 10.
  4. D
    The interface on SW2 enters an err-disabled state because dynamic trunk negotiation automatically enables Spanning Tree PortFast on trunk links.

Answer

The trunk link negotiates successfully via DTP, but SW2 drops the VLAN 30 broadcast frame because VLAN 30 is absent from SW2's allowed VLAN list.
Dynamic Trunking Protocol (DTP) successfully forms an 802.1Q trunk link when one side is configured as 'dynamic auto' and the other as 'dynamic desirable'. However, VLAN allowed lists on trunk interfaces filter traffic independently on each switch. Because SW1 sends a tagged VLAN 30 frame and SW2 only permits VLANs 10 and 20 on its trunk interface, SW2 drops the VLAN 30 frame upon ingress.

Step-by-Step Solution

1
Analyze DTP negotiation state between SW1 and SW2.
SW1 is in dynamic auto mode and SW2 is in dynamic desirable mode. Dynamic desirable actively initiates negotiation, while dynamic auto responds to negotiation requests. This results in an operational 802.1Q trunk.
DTP mode compatibility rules dictate that auto + desirable successfully forms a trunk link.
2
Evaluate frame tagging and egress behavior on SW1 for VLAN 30.
Since VLAN 30 is not the native VLAN (which is VLAN 10), SW1 encapsulates the broadcast frame with an 802.1Q tag for VLAN 30 and forwards it out GigabitEthernet0/2.
802.1Q trunking tags all non-native VLAN traffic passing across the trunk link.
3
Evaluate ingress filtering on SW2 for the incoming tagged VLAN 30 frame.
SW2 receives the tagged VLAN 30 frame. SW2 checks its local trunk allowed VLAN list (`10,20`). Because VLAN 30 is not in SW2's allowed list, SW2 drops the frame at ingress.
Trunk allowed VLAN pruning is enforced locally by receiving switches to restrict unneeded broadcast domains.

Key Concept

802.1Q Trunking DTP Negotiation and Ingress Allowed VLAN Pruning Logic
Estimated Time:1m 15s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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