N10-009 Question 127
Single answerVirtual Extensible Local Area Network (VXLAN) / Data center interconnect (DCI), Layer 2 encapsulationA company is expanding a private cloud across two data centers and must move several virtual machines between sites without changing their IP addresses. The application team also requires that the same VLANs appear in both locations, even though the data centers are connected through an existing Layer 3 IP WAN. The network administrator wants a solution that extends Layer 2 segments across the routed network while avoiding the scalability limits of traditional VLAN-only designs. Which technology best meets these requirements?
- A
VXLAN overlay between the data centers, using Layer 2 frames encapsulated in UDP across the Layer 3 IP network
- B
802.1Q trunking across the WAN router interfaces so the same VLAN IDs can pass natively between sites
- C
A routed GRE tunnel carrying only Layer 3 traffic between the data centers
- D
NAT between the data centers so the virtual machines can keep their applications reachable without extending Layer 2
Show answer and explanation
Correct answer: A
Explanation
The best answer is VXLAN because it was created to address modern virtualization and data center scaling needs. In a DCI scenario, organizations often need to stretch Layer 2 networks between sites over a routed IP transport. VXLAN does this by encapsulating Ethernet frames in UDP, allowing Layer 2 communication to traverse a Layer 3 underlay. This supports workload mobility and consistent addressing between data centers. It also avoids the traditional VLAN scaling limitation of 4094 VLANs by using VNIs, which provide up to 16 million logical segments. This aligns with common vendor documentation and data center design guidance from sources such as RFC 7348, which defines VXLAN, and enterprise best practices for overlay networking in virtualized environments.
- A. Correct.
Correct. VXLAN is specifically designed to extend Layer 2 adjacency over a Layer 3 underlay by encapsulating Ethernet frames inside UDP packets. This makes it well suited for data center interconnect scenarios where workloads need to retain IP addressing and remain in the same logical broadcast domain across sites. VXLAN also improves scalability compared with traditional VLANs by using a 24-bit VXLAN Network Identifier (VNI), which supports far more segments than the 12-bit VLAN ID space.
- B. Incorrect.
Incorrect. Standard 802.1Q trunking extends VLAN tags across Layer 2 links, but it does not solve the problem of traversing a routed Layer 3 WAN by itself. WAN router interfaces do not simply forward Ethernet trunks end-to-end as if they were a single switched domain. Choosing this option reflects the common misconception that VLAN tagging alone can extend Layer 2 across an IP-routed network.
- C. Incorrect.
Incorrect. GRE can tunnel traffic across an IP network, but this option specifically says it is carrying only Layer 3 traffic. The requirement is to preserve Layer 2 segments and allow the same VLAN-backed workloads to exist across both sites. A basic routed GRE design does not inherently provide the Layer 2 overlay function and segmentation scale that VXLAN provides for DCI use cases.
- D. Incorrect.
Incorrect. NAT can help with address translation and application reachability, but it does not extend Layer 2 domains or preserve the same VLAN presence across sites. In fact, using NAT would typically work against the requirement to move virtual machines without changing their IP addresses within the same logical network.