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Zorluk: ZorSwitching Concepts and VLANs

A network host connected to access port FastEthernet0/1 (assigned to VLAN 15) transmits an Ethernet frame intended for a target host. The destination MAC address of this frame is currently absent from the switch's Content Addressable Memory (CAM) table. Port FastEthernet0/2 is an access port in VLAN 15, port FastEthernet0/3 is an access port in VLAN 20, and port GigabitEthernet0/1 is an IEEE 802.1Q trunk port allowing all VLANs. How does the switch handle and forward this unknown unicast frame?

  1. The switch floods the frame out all active interfaces belonging to VLAN 15 (including FastEthernet0/2 and trunk port GigabitEthernet0/1 with an 802.1Q tag), while excluding the ingress port FastEthernet0/1 and VLAN 20 port FastEthernet0/3.Cevap
  2. B
    The switch drops the frame immediately because Layer 2 switches require an established ARP cache entry before forwarding traffic to unknown MAC addresses.
  3. C
    The switch floods the frame out all operational access ports across all configured VLANs to ensure reachability, but omits trunk ports to prevent switching loops.
  4. D
    The switch forwards the frame exclusively out trunk port GigabitEthernet0/1 as untagged traffic, assuming unknown destination hosts reside on neighboring switches.

Cevap

The switch floods the frame out all active interfaces assigned to VLAN 15 (including FastEthernet0/2 and trunk port GigabitEthernet0/1 with an 802.1Q tag for VLAN 15), excluding the receiving port FastEthernet0/1 and FastEthernet0/3 in VLAN 20.
When a Layer 2 switch receives an Ethernet frame with a destination MAC address not present in its CAM table, it performs unknown unicast flooding. This operation floods the frame out every active port within the same VLAN except the port on which the frame was received. Furthermore, trunk links carrying that VLAN will receive the frame with an 802.1Q tag attached so the downstream switch maintains proper VLAN identification.

Adım Adım Çözüm

1
Analyze the frame type and CAM table status.
The destination MAC address is missing from the switch's MAC address table, making this an 'unknown unicast' frame.
Layer 2 switches forward known unicast frames directly to specific ports, but must flood unknown unicast frames to ensure delivery.
2
Determine the broadcast domain boundaries for the ingress port.
The ingress port FastEthernet0/1 is assigned to VLAN 15.
VLANs define Layer 2 broadcast domain boundaries; flooding is strictly limited to ports associated with the ingress frame's VLAN.
3
Identify valid outgoing egress ports.
FastEthernet0/2 (VLAN 15 access port) and GigabitEthernet0/1 (trunk port allowing VLAN 15) are valid egress ports. FastEthernet0/1 (ingress port) and FastEthernet0/3 (VLAN 20) are excluded.
Split horizon rules prevent sending the frame back out the ingress port, and VLAN segmentation prevents sending frames across VLAN boundaries.
4
Evaluate trunk port tagging behavior.
The frame is tagged with an 802.1Q header containing VLAN ID 15 before being sent across trunk port GigabitEthernet0/1.
Non-native VLAN traffic forwarded across an 802.1Q trunk link must be tagged so the receiving switch can identify its target VLAN.

Anahtar Kavram

Layer 2 Unknown Unicast Flooding & VLAN Segmentation
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Konular

Switching Concepts and VLANs

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