Two Cisco Catalyst switches, SW1 and SW2, are connected via their GigabitEthernet0/1 interfaces. SW1 is configured with `switchport mode dynamic desirable` and `switchport trunk native vlan 99`. SW2 is configured with `switchport mode dynamic auto` and retains its default native VLAN 1. Assuming default IEEE 802.1Q trunking behavior, what is the resulting operational state of this interswitch link?
- A trunking operational mode is successfully established via DTP negotiation, but PVST+ places VLAN 1 and VLAN 99 into a blocking state on the link due to a PVID inconsistency.Cevap
- BDTP negotiation fails to form a trunk link because native VLAN configuration parameters must match across both switch ports prior to trunk establishment.
- CA trunk link forms and untagged traffic from VLAN 99 on SW1 is transparently delivered to VLAN 1 on SW2 without any Spanning Tree Protocol port state restrictions.
- DTrunking is established, but PortFast automatically activates on GigabitEthernet0/1 to bypass native VLAN verification checks and prevent frame tagging errors.
Cevap
A trunking operational mode is successfully established via DTP negotiation, but PVST+ places VLAN 1 and VLAN 99 into a blocking state on the link due to a PVID inconsistency.
Dynamic Trunking Protocol (DTP) operates independently of native VLAN configurations, allowing a port in dynamic desirable mode to successfully negotiate an operational 802.1Q trunk with a port in dynamic auto mode. However, Cisco PVST+ sends Spanning Tree BPDUs with VLAN tagging metadata (SSTP). When the switches receive BPDUs indicating different native VLAN IDs (VLAN 99 vs VLAN 1), PVST+ flags a PVID inconsistency and places VLAN 1 and VLAN 99 into a blocking state on the interface to prevent unauthorized cross-VLAN data leakage and loops.
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Anahtar Kavram
802.1Q Native VLAN Mismatch and PVST+ PVID Inconsistency Protection
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