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Zorluk: Çok zorSwitching Concepts and VLANs

A Layer 2 switch receives an untagged Ethernet frame on access port FastEthernet 0/1, which is assigned to VLAN 20. The destination MAC address of the frame is not present in the switch's CAM (MAC address) table. Switch port FastEthernet 0/24 is configured as an 802.1Q trunk port with a native VLAN of 20 and allowed VLANs 10, 20, and 30. Switch port FastEthernet 0/12 is an access port in VLAN 10, and FastEthernet 0/13 is an access port in VLAN 20. How will the switch process and forward this frame?

  1. The switch floods the frame untagged out FastEthernet 0/13 and untagged out trunk port FastEthernet 0/24, while omitting FastEthernet 0/12.Cevap
  2. B
    The switch encapsulates the frame with an 802.1Q VLAN 20 tag before forwarding it out trunk port FastEthernet 0/24, and sends it untagged out FastEthernet 0/13.
  3. C
    The switch floods the frame out all operational interfaces including FastEthernet 0/12, FastEthernet 0/13, and FastEthernet 0/24, because unknown unicast frames ignore VLAN boundaries.
  4. D
    The switch inspects the Layer 3 IP header to determine the destination subnet and forwards the frame out FastEthernet 0/12 after modifying the VLAN tag.

Cevap

The switch floods the frame untagged out FastEthernet 0/13 and untagged out trunk port FastEthernet 0/24, while omitting FastEthernet 0/12.
The incoming frame enters on access port FastEthernet 0/1 in VLAN 20. When a switch receives a frame with an unmapped destination MAC address (unknown unicast), it floods the frame to every active interface within that specific VLAN except the ingress port. FastEthernet 0/13 is in VLAN 20, so it receives the frame untagged. Trunk port FastEthernet 0/24 allows VLAN 20, but since VLAN 20 is configured as the native VLAN on that trunk, 802.1Q rules dictate that the frame is transmitted without a VLAN header tag. FastEthernet 0/12 belongs to VLAN 10 and is isolated from VLAN 20 traffic.

Adım Adım Çözüm

1
Determine the broadcast domain (VLAN) of the incoming frame.
The untagged frame arrives on access port FastEthernet 0/1, which is assigned to VLAN 20. The frame is internally associated with VLAN 20.
Access ports assign untagged ingress frames to their configured port VLAN ID (PVID).
2
Evaluate the switch forwarding rule for an unknown destination MAC address.
Because the destination MAC address is missing from the CAM table, the frame is classified as unknown unicast and must be flooded out all ports belonging to VLAN 20 except ingress port FastEthernet 0/1.
Layer 2 switches perform unknown unicast flooding within the source VLAN boundary.
3
Determine tagging behavior for egress ports.
Port FastEthernet 0/13 is an access port in VLAN 20, so the frame egresses untagged. Port FastEthernet 0/24 is a trunk carrying VLAN 20 as its native VLAN, so the frame egresses untagged. Port FastEthernet 0/12 is in VLAN 10 and is excluded entirely.
802.1Q trunks do not tag frames associated with the designated native VLAN.

Anahtar Kavram

Unknown Unicast Flooding and IEEE 802.1Q Native VLAN Egress Tagging Rules
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Konular

Switching Concepts and VLANs

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