N10-009 Question 126
Single answerVirtual Extensible Local Area Network (VXLAN) / Data center interconnect (DCI), Layer 2 encapsulationA company is connecting two data centers over an IP-based data center interconnect (DCI). Several legacy application servers must stay in the same Layer 2 broadcast domain because they use static IP addressing and are sensitive to subnet changes. The network team wants to extend Layer 2 segments between sites without relying on large-scale Spanning Tree across the WAN. Which technology is the BEST fit for this requirement?
- A
Configure VXLAN to encapsulate Layer 2 Ethernet frames inside Layer 3 UDP packets across the DCI
- B
Deploy NAT between the data centers so both sites can reuse the same VLAN IDs without extending Layer 2
- C
Increase the STP bridge priority on the core switches so a single root bridge controls both data centers
- D
Replace the WAN routers with unmanaged Layer 2 switches to preserve the original Ethernet frames end to end
Show answer and explanation
Correct answer: A
Explanation
The best answer is VXLAN because it addresses a common DCI requirement: extending Layer 2 segments across a routed IP network. VXLAN uses MAC-in-UDP encapsulation so Ethernet frames can traverse a Layer 3 underlay, which is why it is widely used in modern data centers for workload mobility and Layer 2 extension. This is preferable to stretching traditional Layer 2 domains with Spanning Tree over long-distance links, which is generally discouraged due to larger failure domains and slower or more complex convergence behavior. Network+ candidates should recognize the practical distinction between technologies that preserve Layer 2 adjacency, such as VXLAN, and technologies like NAT or STP tuning that do not meet the same requirement. This aligns with common vendor and industry guidance on overlay networking and DCI best practices, including documentation from Cisco, VMware, and IETF VXLAN specifications.
- A. Correct.
Correct. VXLAN is specifically designed to extend Layer 2 networks over a Layer 3 underlay by encapsulating Ethernet frames in UDP/IP packets. This makes it well suited for data center interconnect scenarios where workloads must remain in the same subnet or broadcast domain across sites. VXLAN also helps avoid stretching traditional Layer 2 domains directly over the WAN with large Spanning Tree topologies.
- B. Incorrect.
Incorrect. NAT changes Layer 3 addressing behavior; it does not extend a Layer 2 broadcast domain. Even if VLAN IDs are reused locally at each site, NAT would not allow the legacy servers to remain on the same Layer 2 segment or preserve protocols that depend on Layer 2 adjacency.
- C. Incorrect.
Incorrect. Extending STP across multiple data centers is generally not the preferred design for DCI because it can increase convergence concerns, fault domain size, and operational risk. Changing bridge priority only influences root bridge selection; it does not solve the fundamental need to carry Layer 2 traffic efficiently over a Layer 3 transport.
- D. Incorrect.
Incorrect. Replacing WAN routers with unmanaged Layer 2 switches is not a practical or scalable DCI design. WAN/DCI links are typically IP-routed, and unmanaged switches would remove routing control, visibility, and resiliency features. This option also ignores distance, carrier handoff, and operational limitations common in inter-data-center designs.