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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

A network administrator is troubleshooting interswitch trunking between SW-Core1 and SW-Access1. The administrator issues the `show interfaces trunk` command on SW-Core1 and observes the following output:

PortModeEncapsulationStatusNative vlan
Gi0/24on802.1qtrunking10
PortVlans allowed on trunk
Gi0/241-4094
PortVlans allowed and active in management domain
Gi0/241,10,20,30
PortVlans in spanning tree forwarding state and not pruned
Gi0/241,10,30

Users in VLAN 20 connected to SW-Access1 report complete loss of connectivity to resources in VLAN 20 connected to SW-Core1, whereas users in VLAN 10 and VLAN 30 report normal operation. Based on the command output, what is the cause of the communication failure for VLAN 20?

  1. VLAN 20 is not in the Spanning Tree Protocol forwarding state (or has been pruned) on interface GigabitEthernet0/24.Answer
  2. B
    A native VLAN mismatch exists because the remote switch interface has assigned VLAN 20 as its native VLAN.
  3. C
    PortFast was enabled on interface GigabitEthernet0/24, causing the port to drop tagged frames for VLAN 20.
  4. D
    CDP neighbor TLV negotiation failed, causing the switch to automatically disable VLAN 20 frame tagging.

Answer

VLAN 20 is missing from the Spanning Tree Protocol forwarding state list on the trunk interface, preventing frames in VLAN 20 from being forwarded across the link.
The correct answer identifies that VLAN 20 is absent from the final section of the output: 'Vlans in spanning tree forwarding state and not pruned'. For traffic to pass across an 802.1Q trunk, a VLAN must be allowed on the trunk, active in the VLAN database, and in the STP forwarding state.

Step-by-Step Solution

1
Analyze the 'Vlans allowed on trunk' field in the CLI output.
Interface GigabitEthernet0/24 allows VLANs 1-4094, meaning no explicit manual pruning (`switchport trunk allowed vlan`) is blocking VLAN 20.
Verifies that trunk port configuration permits VLAN 20 traffic.
2
Analyze the 'Vlans allowed and active in management domain' field.
VLAN 20 is listed alongside VLAN 1, 10, and 30, confirming VLAN 20 exists in the local VLAN database and is active.
Rules out local VLAN creation or state issues on SW-Core1.
3
Compare active management VLANs against 'Vlans in spanning tree forwarding state and not pruned'.
VLANs 1, 10, and 30 are forwarding, but VLAN 20 is absent from this list.
If a VLAN is missing from the final STP forwarding section, traffic for that VLAN is either blocked by STP or dynamic VTP pruning, preventing frame transmission across the link.

Key Concept

802.1Q Trunk Operational Verification and STP Per-VLAN Forwarding States
Estimated Time:1m 30s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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