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

During the deployment of an enterprise Cisco Software-Defined Access (SD-Access) fabric, a network engineer must ensure that all fabric edge and control plane switches can exchange IP packets directly. Traditional Layer 3 routing protocols such as OSPF or IS-IS are configured across the physical switch interconnects to establish transport reachability for tunnel endpoints. Which architectural layer is defined by this underlying physical IP routing infrastructure?

  1. The underlay networkAnswer
  2. B
    The overlay network
  3. C
    The fabric control plane
  4. D
    The management plane

Answer

The underlay network
The underlay network consists of the physical routers, switches, and links operating traditional Layer 3 interior gateway protocols (such as IS-IS or OSPF). Its primary purpose in an SDN fabric architecture is to supply robust, highly available IP reachability between all fabric nodes (VTEPs/RLOCs) so that overlay tunnels can be established.

Step-by-Step Solution

1
Analyze the role of physical IP routing in software-defined network architectures.
Identify that physical switches and Layer 3 routing protocols establish basic IP connectivity between network devices.
Before virtual tunnels can form, physical devices must be able to reach each other via IP.
2
Differentiate between underlay, overlay, and control plane layers.
Confirm that the physical topology and routing protocol layer (OSPF/IS-IS/BGP) is called the underlay network.
The underlay provides the foundational IP transport upon which overlay encapsulation protocols (like VXLAN) operate.

Key Concept

Software-Defined Architecture: Underlay vs. Overlay
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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