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Zorluk: OrtaVLAN Configuration and 802.1Q Trunking

A network administrator is setting up an 802.1Q trunk link connecting two switches, SW-Dist01 and SW-Access01, on interface GigabitEthernet1/0/24.

On SW-Dist01, the interface is configured as follows:

switchport mode trunk
switchport trunk native vlan 50
switchport trunk allowed vlan 10,20,50

On SW-Access01, the interface is configured as follows:

switchport mode trunk
switchport trunk native vlan 1
switchport trunk allowed vlan 10,20,50

Based on these configurations, which of the following operational outcomes will occur on this link? (Select TWO)

  1. Untagged frames transmitted from VLAN 50 on SW-Dist01 will be received into VLAN 1 on SW-Access01, creating cross-VLAN traffic leakage.Cevap
  2. Cisco Discovery Protocol (CDP) or log monitoring services will generate native VLAN mismatch warning messages on both switches.Cevap
  3. C
    Tagged frames belonging to VLAN 10 and VLAN 20 will be dropped immediately across the trunk link.
  4. D
    Dynamic Trunking Protocol (DTP) will automatically resolve the discrepancy by reconfiguring SW-Access01 native VLAN to 50.

Cevap

Untagged frames transmitted from VLAN 50 on SW-Dist01 will be received into VLAN 1 on SW-Access01 (causing cross-VLAN traffic leakage), and system management protocols (like CDP) will generate native VLAN mismatch warnings on both switches.
On an 802.1Q trunk, traffic matching the local native VLAN is forwarded untagged. When SW-Dist01 sends untagged VLAN 50 frames, SW-Access01 receives them without a tag and classifies them into its own native VLAN (VLAN 1), causing cross-VLAN traffic leakage. Furthermore, protocols such as CDP exchange VLAN configuration metadata and trigger explicit native VLAN mismatch logs when opposing ends do not match.

Adım Adım Çözüm

1
Analyze frame egress behavior on SW-Dist01 for native VLAN 50
SW-Dist01 sends traffic originating from VLAN 50 across the 802.1Q trunk link untagged because native VLAN traffic is sent without an 802.1Q header tag by default.
802.1Q trunking rules dictate that traffic matching the port's configured native VLAN is sent unencapsulated.
2
Analyze frame ingress behavior on SW-Access01 for untagged frames
SW-Access01 receives the untagged frames and places them directly into its own native VLAN, which is VLAN 1.
An 802.1Q switch port assigns all incoming untagged frames to whichever VLAN ID is configured as its native VLAN.
3
Evaluate protocol monitoring and log detection mechanisms
Neighbor discovery protocols like CDP detect the discrepancy (VLAN 50 vs. VLAN 1) and periodically generate native VLAN mismatch alerts in the console/syslog logs.
CDP advertises local native VLAN configurations in management frames to detect misconfigurations.

Anahtar Kavram

802.1Q Native VLAN Operation and Mismatch Behavior
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Konular

VLAN Configuration and 802.1Q Trunking

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