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Difficulty: HardInterswitch Connectivity and Trunking (802.1Q)

An engineer captures traffic on a point-to-point trunk link between two switches. Switch-1 is configured with `switchport trunk native vlan 10`, while Switch-2 is configured with `switchport trunk native vlan 20`. When Switch-1 transmits untagged frames originating from VLAN 10 across the trunk link, how does Switch-2 process these frames upon ingress, and what operational issue occurs?

  1. Switch-2 assigns the received untagged frames to VLAN 20, resulting in cross-VLAN traffic leakage, while CDP continuously reports a native VLAN mismatch.Answer
  2. B
    Switch-2 drops the untagged frames because their 802.1Q tag header contains a VLAN ID that does not match VLAN 20.
  3. C
    Switch-2 places the trunk interface into an error-disabled state because receiving untagged frames violates STP PortFast trunk settings.
  4. D
    Switch-2 uses the CDP native VLAN TLV to dynamically reconfigure its local trunk interface native VLAN to match VLAN 10.

Answer

Switch-2 assigns the received untagged frames to VLAN 20, resulting in cross-VLAN traffic leakage, while CDP continuously reports a native VLAN mismatch.
Under 802.1Q trunking rules, native VLAN frames are transmitted untagged. When Switch-1 forwards VLAN 10 frames across the trunk without an 802.1Q tag, Switch-2 accepts the untagged frames and assigns them to its own configured native VLAN (VLAN 20). This leads to unintended cross-VLAN traffic leakage between VLAN 10 and VLAN 20. Additionally, CDP detects this discrepancy by exchanging native VLAN details in TLVs and regularly issues syslog warnings.

Step-by-Step Solution

1
Analyze Switch-1 egress processing for native VLAN 10
Frames originating from VLAN 10 are transmitted untagged across the 802.1Q trunk because VLAN 10 matches Switch-1's native VLAN configuration.
According to IEEE 802.1Q specifications, frames belonging to the configured native VLAN are sent across a trunk link without inserting a 4-byte VLAN tag header.
2
Analyze Switch-2 ingress processing for untagged frames
Switch-2 receives the untagged frames and maps them directly into its own native VLAN, which is VLAN 20.
When an 802.1Q trunk interface receives an untagged frame, it automatically assigns that frame to whichever VLAN is designated as native on that local receiving interface.
3
Determine operational impact and diagnostic behavior
Cross-VLAN traffic leakage occurs between VLAN 10 on Switch-1 and VLAN 20 on Switch-2, and CDP logs native VLAN mismatch notifications.
A native VLAN mismatch breaks logical isolation between different VLANs and triggers warning messages via CDP native VLAN TLV exchange.

Key Concept

802.1Q Native VLAN Ingress Processing and Mismatch Behavior
Estimated Time:2m 0s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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