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Zorluk: Çok zorVLAN Configuration and 802.1Q Trunking

A network administrator is inspecting the interface configurations on two interconnecting enterprise switches, SW-CORP-A and SW-CORP-B. The partial configuration snippet for interface GigabitEthernet1/0/12 on SW-CORP-A is shown below:

interface GigabitEthernet1/0/12
switchport mode trunk
switchport trunk native vlan 50
switchport trunk allowed vlan 10,20,30

The corresponding interconnecting interface GigabitEthernet1/0/12 on SW-CORP-B is configured as follows:

interface GigabitEthernet1/0/12
switchport mode trunk
switchport trunk native vlan 10
switchport trunk allowed vlan 10,20,50

If an untagged broadcast frame originating from an end-host assigned to VLAN 50 arrives at SW-CORP-A, what will happen to the frame when SW-CORP-A attempts to process it for transmission across interface GigabitEthernet1/0/12?

  1. The frame is dropped at the egress interface of SW-CORP-A because VLAN 50 is excluded from the trunk's allowed VLAN list.Cevap
  2. B
    The frame is transmitted untagged across the link and mapped directly into VLAN 10 upon arriving at SW-CORP-B due to the native VLAN mismatch.
  3. C
    The frame is encapsulated with an 802.1Q tag for VLAN 50 and routed to VLAN 10 via Layer 3 switch processing on SW-CORP-B.
  4. D
    The frame is transmitted untagged across the trunk link and successfully placed into VLAN 50 on SW-CORP-B because native VLAN traffic bypasses allowed VLAN restrictions.

Cevap

The frame is dropped at the egress interface of SW-CORP-A because VLAN 50 is excluded from the trunk's allowed VLAN list.
On IEEE 802.1Q trunk interfaces, the allowed VLAN list ('switchport trunk allowed vlan') defines which VLANs are permitted to send and receive frames across the link. Omitting a VLAN from this list prevents all frame transmission for that VLAN over the trunk. Even though VLAN 50 is specified as the native VLAN on SW-CORP-A, its exclusion from the allowed VLAN list (which only permits VLANs 10, 20, and 30) causes SW-CORP-A to drop any egress frames for VLAN 50 at interface GigabitEthernet1/0/12.

Adım Adım Çözüm

1
Examine the trunk interface configuration on SW-CORP-A for GigabitEthernet1/0/12.
The interface is set to trunk mode with 'switchport trunk native vlan 50' and 'switchport trunk allowed vlan 10,20,30'.
Determining both the native VLAN setting and the allowed VLAN list is required to evaluate frame forwarding logic.
2
Evaluate the interaction between the native VLAN configuration and the allowed VLAN list on SW-CORP-A.
VLAN 50 is configured as the native VLAN, but VLAN 50 is NOT included in the allowed VLAN list (10, 20, 30).
On 802.1Q trunks, the allowed VLAN list acts as an absolute filter for egress and ingress traffic.
3
Determine the frame handling behavior for ingress VLAN 50 traffic on SW-CORP-A exiting out GigabitEthernet1/0/12.
Because VLAN 50 is absent from the allowed VLAN list, SW-CORP-A drops all egress frames belonging to VLAN 50 at the interface.
Excluding a VLAN from the allowed list prevents any frames for that VLAN from traversing the trunk, overriding native VLAN tagging/untagging parameters.

Anahtar Kavram

802.1Q Trunk Allowed VLAN Filtering vs Native VLAN Configuration
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Konular

VLAN Configuration and 802.1Q Trunking

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