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

A network administrator is evaluating the functions of underlay and overlay components within a Cisco Software-Defined Access (SD-Access) fabric deployment. Which two statements accurately describe the responsibilities and operational relationships of the underlay and overlay networks? (Select two.)

  1. The underlay network relies on traditional routing protocols, such as OSPF or IS-IS, to establish unicast IP reachability between physical fabric nodes.Answer
  2. The overlay network utilizes Virtual Extensible LAN (VXLAN) encapsulation to transport virtualized Layer 2 and Layer 3 user traffic across the underlay infrastructure.Answer
  3. C
    The underlay network uses LISP to dynamically map virtual host endpoint identifiers (EIDs) to physical routing locators (RLOCs).
  4. D
    The overlay network eliminates the requirement for physical IP routing between fabric edge nodes.
  5. E
    The underlay network performs packet encapsulation by adding MAC-in-UDP headers to client frames at access ports.

Answer

The correct statements are that the underlay network relies on traditional routing protocols (like OSPF or IS-IS) to establish IP reachability between physical fabric nodes, and the overlay network utilizes VXLAN encapsulation to transport Layer 2 and Layer 3 user traffic across the underlay infrastructure.
In software-defined fabric architectures (like Cisco SD-Access), the underlay provides basic physical connectivity and IP reachability using traditional IGPs (such as OSPF or IS-IS). The overlay runs on top of the underlay and uses VXLAN to encapsulate Layer 2 and Layer 3 frames into UDP packets, allowing virtual network separation independently of physical cabling.

Step-by-Step Solution

1
Analyze the role of the underlay network in SDN architectures.
The underlay consists of physical switches, routers, and links running standard Layer 3 IP routing protocols (e.g., OSPF, IS-IS, BGP) to ensure IP reachability among fabric devices.
Without underlay IP reachability, overlay tunnels cannot be established between endpoints.
2
Analyze the role of the overlay network and its protocols.
The overlay is a virtual network built on top of the underlay that uses encapsulation (such as VXLAN) for data plane transport and mapping protocols (such as LISP) for control plane tracking.
Overlay networks decoupled logical user segments (VNFs/VLANs) from the underlying physical topology.
3
Evaluate the choices to select the two correct architectural statements.
Statements highlighting IGP IP reachability for underlay and VXLAN encapsulation for overlay are correct.
Misattributing LISP or VXLAN to the underlay confuses overlay data/control plane mechanics with physical transport.

Key Concept

Underlay vs. Overlay Architecture in SDN Fabrics
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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