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Difficulty: HardSwitching Concepts and VLANs

Host A (IP address 10.1.10.45/24) is connected to an access port assigned to VLAN 10 on a Layer 2 switch. Host B (IP address 10.1.20.80/24) is connected to another access port assigned to VLAN 20 on the same switch. Host A attempts to send a packet to Host B, causing Host A to broadcast an ARP request for Host B's IP address. No Layer 3 router or Layer 3 switch interface is configured on the network. How does the Layer 2 switch process the incoming ARP broadcast frame?

  1. The switch floods the ARP broadcast frame exclusively to operational ports assigned to VLAN 10, preventing Host B from receiving the request.Answer
  2. B
    The switch floods the ARP broadcast frame to all operational ports across all VLANs because ARP requests are Layer 2 broadcasts.
  3. C
    The switch inspects the target IP address in the payload, converts the ARP frame into a Layer 3 packet, and routes it directly to VLAN 20.
  4. D
    The switch untags the frame and forwards it out of the native VLAN port to bridge communication between VLAN 10 and VLAN 20.

Answer

The switch floods the ARP broadcast frame exclusively to operational ports assigned to VLAN 10, preventing Host B from receiving the request.
VLANs create distinct Layer 2 broadcast domains. When Host A generates an ARP broadcast request on VLAN 10, the Layer 2 switch floods the frame only out of ports assigned to VLAN 10. Since Host B is on VLAN 20 and no Layer 3 routing device exists to route traffic between the subnets, Host B will never receive the ARP request.

Step-by-Step Solution

1
Identify the ingress port and VLAN membership of the sender.
Host A is connected to an access port assigned to VLAN 10, placing its generated frames inside VLAN 10's broadcast domain.
Layer 2 switches associate incoming frames on access ports with the port's assigned VLAN ID.
2
Analyze frame destination MAC address and switch forwarding rules.
An ARP request uses the Layer 2 broadcast destination address (FF:FF:FF:FF:FF:FF). The switch floods broadcast frames to all active ports within the ingress VLAN.
Broadcast frames must be sent to all members of the broadcast domain, but VLAN boundaries strictly limit the scope of the flood.
3
Evaluate inter-VLAN reachability without a Layer 3 gateway.
Because Host B is on VLAN 20 and no router/Layer 3 device is present, the broadcast frame never reaches Host B, and inter-VLAN communication fails.
VLANs act as logically independent switch instances at Layer 2 and require Layer 3 routing (Router-on-a-Stick or Layer 3 switch SVI) to forward traffic between different subnets/VLANs.

Key Concept

VLAN Broadcast Domain Isolation
Estimated Time:1m 30s
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Topics

Switching Concepts and VLANs

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