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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

Two Cisco Catalyst switches, Switch-A and Switch-B, are connected via their GigabitEthernet0/1 interfaces. Switch-A has its interface configured with the command `switchport mode dynamic desirable`, while Switch-B operates using the default Cisco Catalyst DTP configuration (`switchport mode dynamic auto`). Both switches retain all default VLAN parameters. Which of the following statements regarding the operational behavior of this interswitch connection are true? (Select TWO.)

  1. The interface link successfully negotiates an operational 802.1Q trunking state via Dynamic Trunking Protocol (DTP).Answer
  2. Traffic belonging to VLAN 1 is transmitted across the trunk link without an added 802.1Q VLAN header tag.Answer
  3. C
    Frames belonging to VLAN 1 will have a 4-byte 802.1Q tag prepended with a VLAN ID value of 1 prior to transmission.
  4. D
    Spanning Tree Protocol automatically enables PortFast on both trunk ends to bypass listening and learning states.

Answer

The connection successfully negotiates an operational 802.1Q trunk link via DTP, and frames belonging to the default native VLAN (VLAN 1) are transmitted across the trunk untagged.
When combining `switchport mode dynamic desirable` on one end and `switchport mode dynamic auto` on the other, DTP successfully negotiates an operational trunk. Furthermore, because default settings maintain VLAN 1 as the native VLAN, frames associated with VLAN 1 are forwarded across the trunk without receiving a 4-byte 802.1Q VLAN header tag.

Step-by-Step Solution

1
Analyze the Dynamic Trunking Protocol (DTP) mode combination between Switch-A and Switch-B.
Switch-A (`dynamic desirable`) actively sends DTP negotiation frames. Switch-B (`dynamic auto`) listens and agrees to form a trunk upon receiving DTP requests from a desirable port.
DTP operational state rules dictate that desirable + auto results in an operational trunk.
2
Evaluate the 802.1Q native VLAN framing mechanics for default VLAN configuration.
VLAN 1 is the default native VLAN on both switches. Under IEEE 802.1Q standards, frames belonging to the designated native VLAN cross the trunk link untagged.
802.1Q tagging adds a 4-byte header only to non-native VLAN traffic to ensure backward compatibility and minimal processing overhead for native subnet traffic.

Key Concept

802.1Q Trunking and Dynamic Trunking Protocol (DTP) Negotiation
Estimated Time:1m 30s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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