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Zorluk: OrtaL2 Switching Concepts and MAC Table Operation

Host A with MAC address 0011.2233.4455 is connected to interface FastEthernet0/5 assigned to VLAN 20 on switch SW-Central. The MAC address table on SW-Central contains no existing entries for VLAN 20. When Host A transmits an ARP request frame to resolve its default gateway IP address, which operational sequence will SW-Central perform upon receiving the ingress frame?

  1. Learn source MAC 0011.2233.4455 on interface FastEthernet0/5 in VLAN 20, then flood the frame out all active interfaces assigned to VLAN 20 except FastEthernet0/5.Cevap
  2. B
    Learn source MAC 0011.2233.4455 globally across all VLANs, then flood the frame out every operational interface on the switch regardless of VLAN membership.
  3. C
    Forward the frame directly to the default gateway interface without updating the MAC address table, relying on Layer 2 discovery protocol neighbors.
  4. D
    Drop the frame immediately because the destination MAC address FFFF.FFFF.FFFF is not dynamically registered in the VLAN 20 MAC address table.

Cevap

The switch records MAC 0011.2233.4455 on interface FastEthernet0/5 in VLAN 20 and floods the frame out all active interfaces in VLAN 20 except FastEthernet0/5.
When a Layer 2 switch receives an Ethernet frame, it first updates its MAC address table by associating the source MAC address (0011.2233.4455) with the receiving port (FastEthernet0/5) and VLAN (VLAN 20). Next, because an ARP request uses the Ethernet broadcast destination address (FFFF.FFFF.FFFF), the switch floods the frame out every port in VLAN 20 except the ingress interface.

Adım Adım Çözüm

1
Examine the source MAC address of the incoming Ethernet frame.
The source MAC address 0011.2233.4455 is identified on port FastEthernet0/5.
Layer 2 switches perform dynamic MAC address learning by inspecting the source MAC field of all ingress frames.
2
Update the switch MAC address table for VLAN 20.
An entry mapping MAC 0011.2233.4455 to port FastEthernet0/5 in VLAN 20 is created.
MAC table entries are stored per-VLAN to maintain Layer 2 traffic isolation.
3
Examine the destination MAC address of the frame.
The destination MAC address is FFFF.FFFF.FFFF (Ethernet broadcast).
ARP requests use the broadcast destination MAC address so that all devices in the broadcast domain receive the query.
4
Forward the broadcast frame according to Layer 2 switching rules.
The frame is flooded to all active ports belonging to VLAN 20, excluding ingress port FastEthernet0/5.
Broadcast frames must reach all hosts in the same VLAN domain, but split-horizon logic prevents sending the frame back out its ingress port.

Anahtar Kavram

L2 Switch MAC Table Operation and Broadcast Flooding Rules
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