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Difficulty: HardSoftware-Defined Architectures: Overlay, Underlay, and Fabric

In a Cisco Software-Defined Access (SD-Access) solution, an enterprise deploys a Layer 3 routed underlay network using OSPF to establish IP reachability between Routing Locator (RLOC) interfaces. During testing, host data packets transmitted between different fabric edge switches fail whenever the payload size approaches 1500 bytes. Analysis shows that packet payload expansion is occurring because host frames are encapsulated with outer IP and UDP headers before entering the physical transport. Which protocol and architectural layer are responsible for performing this data plane encapsulation?

  1. Virtual Extensible LAN (VXLAN) operating at the overlay layerAnswer
  2. B
    Locator/ID Separation Protocol (LISP) operating at the underlay layer
  3. C
    Open Shortest Path First (OSPF) operating at the fabric control plane layer
  4. D
    Border Gateway Protocol (BGP) operating at the overlay layer

Answer

Virtual Extensible LAN (VXLAN) operating at the overlay layer
The correct option correctly identifies VXLAN operating at the overlay layer. In Cisco SD-Access architectures, VXLAN is employed as the data plane encapsulation mechanism. It encapsulates original Ethernet frames into UDP-based packets (using UDP port 4789) to transport Layer 2 and Layer 3 virtual networks across an IP-only underlay. This encapsulation adds header overhead (typically 50 bytes), requiring the underlay physical network to support an increased MTU (jumbo frames).

Step-by-Step Solution

1
Differentiate between the underlay and overlay architectural components in Cisco SD-Access.
The underlay consists of physical routers/switches and an IGP (such as OSPF or IS-IS) providing IP connectivity between switch loopback interfaces (RLOCs). The overlay represents the virtual network built on top of the underlay.
Understanding the separation of underlay and overlay is necessary to determine which layer manages data encapsulation.
2
Identify the specific roles of LISP and VXLAN in the SD-Access fabric.
LISP operates as the fabric control plane mapping database (mapping EID to RLOC). VXLAN operates as the fabric data plane protocol.
LISP manages location/identity queries while VXLAN encapsulates actual host data traffic into UDP frames.
3
Analyze the cause of payload expansion and MTU issues.
VXLAN wraps the original Layer 2 Ethernet frame inside outer IP, UDP, and VXLAN headers. This adds approximately 50 bytes of overhead, causing frames near 1500 bytes to exceed standard MTU unless jumbo frames are enabled in the underlay.
VXLAN encapsulation at the overlay layer directly causes the payload expansion described in the scenario.

Key Concept

Software-Defined Architecture: Overlay vs Underlay Roles and VXLAN Data Plane Encapsulation
Estimated Time:2m 0s
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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