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Zorluk: Çok zorInterswitch Connectivity and Trunking (802.1Q)

A network engineer connects two Cisco Catalyst switches using interface GigabitEthernet0/12. The interfaces are configured with the following parameters:

Switch_Alpha (GigabitEthernet0/12):
- DTP Mode: `dynamic desirable`
- Trunking Encapsulation: `dot1q`
- Native VLAN: `20`
- Allowed VLANs: `10, 20, 30`

Switch_Beta (GigabitEthernet0/12):
- DTP Mode: `dynamic auto`
- Trunking Encapsulation: `dot1q`
- Native VLAN: `1`
- Allowed VLANs: `10, 30, 40`

Which two operational outcomes will occur on this interconnect link? (Select two.)

  1. The link will successfully negotiate an operational 802.1Q trunk state.Cevap
  2. Cisco Discovery Protocol (CDP) will generate error messages warning of a native VLAN mismatch.Cevap
  3. C
    Dynamic Trunking Protocol (DTP) negotiation will fail because the allowed VLAN lists on opposing switch interfaces do not match.
  4. D
    Switch_Beta will automatically insert an 802.1Q tag into outgoing frames belonging to VLAN 1 to resolve the mismatch.

Cevap

The link will successfully negotiate an operational 802.1Q trunk state, and Cisco Discovery Protocol (CDP) will generate error messages warning of a native VLAN mismatch.
DTP mode 'dynamic desirable' actively negotiates trunking with 'dynamic auto', resulting in an operational trunk link. Additionally, because the two switches have conflicting native VLAN configurations (VLAN 20 vs VLAN 1), CDP exchange mechanisms detect the PVID mismatch and generate warning log alerts.

Adım Adım Çözüm

1
Analyze DTP negotiation state between the two switches
Dynamic desirable initiates trunk negotiation while dynamic auto responds to requests. The combination successfully forms an operational 802.1Q trunk link.
DTP requires at least one side to be in desirable or trunk mode to actively negotiate a trunk.
2
Evaluate the native VLAN configuration on both interface endpoints
Switch_Alpha treats VLAN 20 as native, while Switch_Beta treats VLAN 1 as native.
Mismatched native VLAN definitions do not prevent DTP state completion, but cause CDP to detect PVID mismatches and log critical warning messages.
3
Verify allowed VLAN filtering rules and frame tagging behavior
Allowed VLAN lists do not restrict DTP state negotiation, and native VLAN frames remain untagged by default on 802.1Q trunks unless global native VLAN tagging is enabled.
802.1Q trunking sends frames belonging to the configured local native VLAN across the link without adding a 4-byte 802.1Q VLAN header.

Anahtar Kavram

802.1Q Trunking DTP Negotiation and Native VLAN Mismatch Behaviors
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Konular

Interswitch Connectivity and Trunking (802.1Q)

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