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Zorluk: OrtaInterswitch Connectivity and Trunking (802.1Q)

A network technician connects interface GigabitEthernet0/1 on Switch-A to interface GigabitEthernet0/1 on Switch-B using an Ethernet link. Switch-A retains its factory default interface configuration of `switchport mode dynamic auto`, while Switch-B is explicitly configured with `switchport mode dynamic desirable`. Which two operational conditions describe the resulting state of this interswitch link?

  1. The link successfully negotiates and operates as an 802.1Q trunk link.Cevap
  2. Frames belonging to the native VLAN traverse the trunk link without an 802.1Q header tag by default.Cevap
  3. C
    The link operational mode defaults to an access port because dynamic auto refuses trunk establishment requests from dynamic desirable ports.
  4. D
    Spanning Tree Protocol PortFast is automatically enabled on the link once trunking negotiation completes.

Cevap

The link negotiates to operate as an 802.1Q trunk, and native VLAN frames cross the trunk link untagged by default.
When configuring interswitch trunking via DTP, combining `dynamic desirable` on one end with `dynamic auto` on the other results in successful 802.1Q trunk negotiation because `desirable` actively initiates the negotiation state while `auto` accepts it. Additionally, under IEEE 802.1Q standard operation, frames destined for the native VLAN (default VLAN 1) are transmitted across the trunk link without an added 802.1Q header tag.

Adım Adım Çözüm

1
Evaluate the Dynamic Trunking Protocol (DTP) negotiation between dynamic auto and dynamic desirable modes.
Dynamic desirable actively initiates DTP negotiation, and dynamic auto listens and agrees to form a trunk. Thus, the operational mode becomes an 802.1Q trunk.
DTP operational logic forms a trunk whenever at least one side actively negotiates (desirable or trunk) and the opposite side is willing to negotiate (auto or desirable).
2
Analyze standard 802.1Q frame encapsulation mechanics for native VLAN traffic.
By default, VLAN 1 is the native VLAN, and standard 802.1Q trunking does not insert a 4-byte 802.1Q tag into frames matching the native VLAN.
IEEE 802.1Q specifies native VLAN traffic as untagged to maintain backwards compatibility with legacy non-trunking Ethernet devices.

Anahtar Kavram

DTP Mode Negotiation and 802.1Q Native VLAN Tagging Mechanics
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Interswitch Connectivity and Trunking (802.1Q)

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