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

A network engineer is auditing a enterprise Software-Defined Access (SD-Access) fabric deployment where endpoint devices in different subnets communicate across the campus network. During a traffic flow analysis, the engineer notes that Layer 2 Ethernet frames are encapsulated into VXLAN packets at the edge switches before transport. Which function represents the primary operational responsibility of the underlying network (underlay) in this architecture?

  1. Providing IP reachability and routing between physical fabric nodes and Tunnel Endpoints (VTEPs)Answer
  2. B
    Encapsulating original Layer 2 frames into VXLAN UDP packets to maintain virtual network isolation
  3. C
    Maintaining the LISP Mapping Database to resolve Endpoint Identifiers (EIDs) to Routing Locators (RLOCs)
  4. D
    Assigning and carrying Security Group Tags (SGTs) to enforce micro-segmentation policy across fabric links

Answer

The primary operational responsibility of the underlay network is providing IP reachability and routing between physical fabric nodes and Tunnel Endpoints (VTEPs).
The underlay network provides the physical infrastructure and Layer 3 unicast routing (using protocols such as IS-IS or OSPF) to establish reliable IP reachability between physical fabric devices and VTEP/RLOC interfaces.

Step-by-Step Solution

1
Define the role of the Underlay network in Software-Defined Architectures
The underlay is the physical infrastructure (switches, routers, links) and Layer 3 routing protocols (e.g., OSPF, IS-IS, BGP) that supply IP connectivity.
Without underlay IP reachability, overlay tunnels cannot be established between endpoints.
2
Differentiate Underlay transport from Overlay encapsulation and control functions
Overlay handles VXLAN encapsulation, LISP EID-to-RLOC mappings, and virtual network segmentations. The underlay only routes standard IP packets between the VTEP/RLOC loopback addresses.
Clear separation between underlay transport and overlay virtual networks is a fundamental SDN concept.

Key Concept

Underlay vs. Overlay Responsibilities in SDN Fabrics
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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