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Zorluk: ZorTroubleshooting VLAN, Trunking, and Switching Issues

A network technician is troubleshooting connectivity for workstations in a newly configured department. The workstations are connected to access ports on Switch-A assigned to VLAN 20. However, none of the hosts can acquire an IP address from the DHCP server located across an 802.1Q trunk link connected to the core switch. The technician issues the following CLI command on Switch-A interface GigabitEthernet0/1:

text
Switch-A# show interfaces trunk

Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1

Port Vlans allowed on trunk
Gi0/1 1-10,30-100

Port Vlans in spanning tree forwarding state and not pruned
Gi0/1 1,10,30,40

Based on the terminal output and symptoms, which configuration issue on interface GigabitEthernet0/1 is preventing communication for hosts on VLAN 20?

  1. VLAN 20 is absent from the trunk interface allowed VLAN list.Cevap
  2. B
    A native VLAN mismatch exists between Switch-A and the core switch trunk port.
  3. C
    Switch-A has an improper Spanning Tree Protocol priority set for VLAN 20, placing the port into blocking mode.
  4. D
    An active duplex mismatch on interface GigabitEthernet0/1 is dropping all VLAN 20 traffic.

Cevap

VLAN 20 is absent from the trunk interface allowed VLAN list.
The correct answer identifies that VLAN 20 is missing from the 802.1Q trunk allowed list. The command output indicates that interface GigabitEthernet0/1 only permits VLAN ranges 1-10 and 30-100. Consequently, frames originating from access ports in VLAN 20 are pruned at the trunk boundary and cannot reach external network services like DHCP.

Adım Adım Çözüm

1
Examine the 'Vlans allowed on trunk' field in the interface trunk output.
The range specified is '1-10,30-100'.
By default, an 802.1Q trunk allows VLANs 1-4094, but explicit filtering using the switchport trunk allowed vlan command restricts allowed traffic.
2
Compare the department VLAN ID with the allowed VLAN list.
VLAN 20 falls outside the allowed ranges (1-10 and 30-100).
When a VLAN ID is omitted from the allowed list, the switch interface discards incoming frames tagged with that VLAN ID and will not forward outgoing frames for that VLAN.
3
Determine the required resolution to restore traffic flow.
Execute 'switchport trunk allowed vlan add 20' on interface GigabitEthernet0/1.
Adding VLAN 20 permits tagged frames for VLAN 20 to transit the trunk link to reach the upstream DHCP server and gateway.

Anahtar Kavram

802.1Q Trunk Allowed VLAN Filtering
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Konular

Troubleshooting VLAN, Trunking, and Switching Issues

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