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Zorluk: ZorInterswitch Connectivity and Trunking (802.1Q)

Two Cisco Catalyst switches, SW1 and SW2, are connected via their GigabitEthernet0/1 interfaces. SW1 is configured with `switchport mode dynamic desirable`, `switchport trunk native vlan 20`, and `switchport trunk allowed vlan 10,20,30`. SW2 is configured with default DTP settings (`switchport mode dynamic auto`), `switchport trunk native vlan 1`, and `switchport trunk allowed vlan 10,20,40`. Which of the following statements accurately describe the operational status and traffic behavior across this interswitch link? (Select TWO.)

  1. The link operational state becomes an 802.1Q trunk because Dynamic Trunking Protocol (DTP) dynamic desirable mode on SW1 successfully negotiates with dynamic auto mode on SW2.Cevap
  2. Traffic transmitted across the link for VLAN 10 is tagged with an explicit 4-byte 802.1Q header containing VLAN ID 10.Cevap
  3. C
    SW1 dynamically encapsulates VLAN 20 frames with a 4-byte 802.1Q tag prior to transmission after detecting that SW2 uses a different native VLAN.
  4. D
    The trunk link negotiation fails completely because dynamic auto mode requires explicit static trunking commands on the opposing interface.

Cevap

The operational state becomes an 802.1Q trunk due to successful DTP negotiation between dynamic desirable and dynamic auto modes, and traffic for VLAN 10 will carry a standard 4-byte 802.1Q tag header across the trunk link.
The combination of dynamic desirable on one end and dynamic auto on the other causes DTP to successfully negotiate an operational 802.1Q trunk. For any tagged VLAN allowed across the link that is not designated as native (such as VLAN 10), 802.1Q inserts a standard 4-byte field into the Ethernet frame header.

Adım Adım Çözüm

1
Analyze Dynamic Trunking Protocol (DTP) operational mode interaction.
SW1 (dynamic desirable) actively sends DTP frames to negotiate trunking. SW2 (dynamic auto) listens and agrees to trunk, resulting in an operational trunk link.
Dynamic desirable + dynamic auto always yields an operational trunk link.
2
Evaluate IEEE 802.1Q tagging behavior for non-native allowed VLANs.
VLAN 10 is allowed on both sides and is non-native for both switches. Frames belonging to VLAN 10 are inserted with an explicit 4-byte 802.1Q header containing the VLAN ID.
Standard 802.1Q trunking tags all non-native VLAN traffic.
3
Evaluate native VLAN processing during mismatches.
SW1 sends VLAN 20 untagged (its native VLAN), while SW2 expects VLAN 1 untagged. Neither switch automatically tags its native VLAN unless globally configured via 'vlan dot1q tag native'. Mismatched native VLAN configuration leads to STP PVID inconsistencies, but does not alter local untagged frame generation.
Native VLAN configuration dictates local untagged frame transmission rules independently on each side.

Anahtar Kavram

802.1Q Interswitch Trunking Negotiation and Frame Tagging Mechanics
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Interswitch Connectivity and Trunking (802.1Q)

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