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Zorluk: Çok zorTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is analyzing console log messages and packet captures between two Layer 2 switches, Switch-A and Switch-B, connected via an 802.1Q trunk on port GigabitEthernet0/1. Switch-A has GigabitEthernet0/1 configured with 'switchport trunk native vlan 10', whereas Switch-B has GigabitEthernet0/1 configured with 'switchport trunk native vlan 20'. Cisco Discovery Protocol (CDP) alerts report a %CDP-4-NATIVE_VLAN_MISMATCH error. Hosts on Switch-A in VLAN 10 report unexpected cross-talk and data leaks to hosts on Switch-B in VLAN 20, while tagged VLAN 30 traffic functions normally. Which of the following correctly explains the underlying root cause of this behavior and the required resolution?

  1. The native VLAN mismatch causes untagged frames sent from Switch-A (VLAN 10) to be received and ingested as untagged frames on Switch-B and assigned to its local native VLAN (VLAN 20); configuring matching native VLAN numbers on both trunk endpoints resolves the cross-VLAN leakage.Cevap
  2. B
    Late collisions caused by a duplex mismatch on GigabitEthernet0/1 are stripping the 802.1Q header tags from VLAN 10 frames prior to arrival at Switch-B; manually setting both trunk ports to full-duplex speed and mode will restore proper tagging.
  3. C
    Layer 2 switches cannot pass traffic between identical VLANs across physical switch boundaries without a Layer 3 inter-VLAN router; installing an external router to perform subinterface routing will resolve the communication issue.
  4. D
    Spanning Tree Protocol (STP) has placed GigabitEthernet0/1 on Switch-A into a blocking state because Switch-A has an STP bridge priority of 61440; decreasing Switch-A's STP priority to 65535 will elect Switch-A as the root bridge and unblock the port.

Cevap

The native VLAN mismatch causes untagged frames sent from Switch-A (VLAN 10) to be received and ingested as untagged frames on Switch-B and assigned to its local native VLAN (VLAN 20); configuring matching native VLAN numbers on both trunk endpoints resolves the cross-VLAN leakage.
In 802.1Q trunking, traffic belonging to the native VLAN is sent untagged across the link. When Switch-A sends VLAN 10 traffic untagged (due to native VLAN 10 configuration), Switch-B receives those untagged frames and implicitly assigns them to its own native VLAN (VLAN 20). This results in untagged broadcast domain leakage across VLAN boundaries. Matching the native VLAN settings across both switches resolves the issue.

Adım Adım Çözüm

1
Analyze the 802.1Q trunking protocol behavior for native VLAN traffic.
IEEE 802.1Q trunks transmit frames belonging to the designated native VLAN without an explicit 4-byte 802.1Q header tag.
Native VLAN frames are unencapsulated for backward compatibility with non-trunking devices.
2
Trace frame egress from Switch-A and ingress on Switch-B.
Switch-A transmits VLAN 10 traffic untagged because VLAN 10 is its native VLAN. Switch-B receives untagged frames on GigabitEthernet0/1 and maps all incoming untagged frames to its own native VLAN, which is VLAN 20.
Trunk interfaces do not carry native VLAN identification metadata inside untagged frame headers.
3
Determine the necessary administrative resolution.
Aligning the native VLAN configurations on both Switch-A and Switch-B (e.g., setting both to VLAN 10 or both to an unused VLAN like VLAN 99) eliminates the native VLAN mismatch log and prevents untagged traffic leakage.
Both switches must agree on the native VLAN ID on trunk links for symmetrical handling of untagged frame encapsulation.

Anahtar Kavram

Native VLAN Mismatch Behavior on IEEE 802.1Q Trunk Links
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Troubleshooting VLAN, Trunking, and Switching Issues

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