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Difficulty: Very hardNetwork Components Roles and Functions

An enterprise data center environment utilizes bare-metal Type 1 hypervisors connected to an upstream physical Layer 3 switch acting as the default gateway for all host subnets. Host-A contains Virtual Machine 1 (VM-1) on VLAN 10 (10.1.10.0/2410.1.10.0/24) and Virtual Machine 2 (VM-2) on VLAN 20 (10.1.20.0/2410.1.20.0/24). The virtual switch (vSwitch) running inside Host-A operates strictly as a Layer 2 virtual device with no internal Layer 3 routing interface configured. When VM-1 sends the initial packet to establish a session with VM-2, which network component makes the forwarding decision to route traffic between these two virtual machines?

  1. The upstream physical Layer 3 switch receives the frame via the trunk link, performs a routing table lookup, and routes the packet back down the trunk to Host-A's vSwitch for delivery to VM-2.Answer
  2. B
    The local Layer 2 vSwitch inspects the destination IP packet header and switches the frame internally between VLAN 10 and VLAN 20 ports without sending traffic out of the physical host interface.
  3. C
    The Type 1 hypervisor directly routes the IP packet between VM-1 and VM-2 within host memory because hosted hypervisors natively execute Layer 3 packet switching without relying on physical network devices.
  4. D
    The CDP/LLDP discovery daemon running on the vSwitch reads the target neighbor's MAC address TLV to establish a direct internal loopback path between the two virtual network interface cards.

Answer

The upstream physical Layer 3 switch receives the frame via the trunk link, performs a routing table lookup, and routes the packet back down the trunk to Host-A's vSwitch for delivery to VM-2.
Because VM-1 and VM-2 are situated on different VLANs (10.1.10.0/2410.1.10.0/24 and 10.1.20.0/2410.1.20.0/24), communication between them requires Layer 3 routing. Since the vSwitch on Host-A is explicitly operating as a Layer 2 switch without L3 capabilities, it cannot route packets between VLANs internally. Therefore, VM-1 sends the packet to its default gateway, causing the frame to exit Host-A via a trunk interface, reach the physical Layer 3 switch where the inter-VLAN routing decision occurs, and return back down the trunk to Host-A for delivery to VM-2.

Step-by-Step Solution

1
Determine the source and destination subnets
VM-1 (10.1.10.0/2410.1.10.0/24) and VM-2 (10.1.20.0/2410.1.20.0/24) belong to distinct IP subnets and VLANs.
Traffic between different IP subnets requires a Layer 3 routing decision (inter-VLAN routing).
2
Evaluate the capabilities of the host's virtual network infrastructure
The local vSwitch operates strictly at Layer 2 and lacks L3 routing interfaces.
A Layer 2 device can only switch frames within the same broadcast domain (VLAN). It cannot rewrite MAC headers or cross VLAN boundaries.
3
Trace the packet path for inter-VLAN routing
VM-1 sends the frame to its default gateway MAC address. The vSwitch forwards the frame up the physical trunk link to the physical Layer 3 switch. The L3 switch routes the packet from VLAN 10 to VLAN 20 and sends it back down the trunk to the vSwitch, which delivers it to VM-2.
Even though both VMs reside on the same hypervisor host, inter-VLAN traffic must traverse the default gateway on the upstream L3 switch ('router-on-a-stick' / external L3 forwarding pattern).

Key Concept

Inter-VLAN Routing and Layer 2 Virtual Switch Boundaries
Estimated Time:2m 0s
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Topics

Network Components Roles and Functions

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