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

An network administrator is designing a fabric-based Software-Defined Access (SD-Access) solution for a remote branch campus. Which two operational characteristics specifically describe the role and function of the underlay network within this architecture? (Select two.)

  1. It provides physical Layer 3 IP reachability between all network devices, such as fabric edge and control plane nodes.Answer
  2. B
    It encapsulates user traffic into Virtual Extensible LAN (VXLAN) headers to maintain virtual network isolation across the campus.
  3. It relies on standard IGP routing protocols (such as IS-IS or OSPF) to transport IP packets between routed interfaces.Answer
  4. D
    It acts as a centralized database mapping Endpoint Identifiers (EIDs) to Routing Locators (RLOCs) for dynamic endpoint tracking.
  5. E
    It manages automated policy enforcement and micro-segmentation using Security Group Tags (SGTs).

Answer

The underlay network provides physical Layer 3 IP reachability between all nodes and relies on standard IGP routing protocols (such as IS-IS or OSPF) to transport IP packets across physical interfaces.
The underlay network forms the physical foundation of an SDN fabric. Its primary purpose is to provide robust, high-availability Layer 3 IP reachability between all participating fabric nodes using standard Interior Gateway Protocols (IGPs) such as IS-IS or OSPF.

Step-by-Step Solution

1
Define the primary responsibility of the underlay network in SDN architectures.
The underlay consists of physical switches, routers, and links configured to ensure basic Layer 3 unicast reachability between all network infrastructure nodes.
Overlay services like VXLAN tunneling and LISP lookup require dependable IP reachability between Tunnel Endpoints (RLOCs).
2
Identify protocols operating at the underlay layer versus the overlay layer.
Underlay uses traditional routing protocols (OSPF, IS-IS, BGP) to transport IP packets, while overlay uses protocols such as VXLAN for encapsulation and LISP for mapping.
Distinguishing between physical IP transport (underlay) and virtualized service encapsulation (overlay) clarifies architectural boundaries.

Key Concept

Underlay vs. Overlay Architecture in Software-Defined Fabrics
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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