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Difficulty: Very hardSoftware-Defined Architectures: Overlay, Underlay, and Fabric

Network architects are evaluating packet encapsulation and routing mechanisms within a modern Cisco Software-Defined Access (SD-Access) fabric architecture. Which two statements correctly differentiate the operational capabilities and responsibilities of the underlay network versus the overlay network? (Select two.)

  1. The underlay network provides underlying physical or logical IP connectivity between fabric nodes using standard routing protocols like OSPF, IS-IS, or BGP to reach Routing Locator (RLOC) addresses.Answer
  2. The overlay network uses VXLAN encapsulation to encapsulate Layer 2 frames and Layer 3 packets into UDP packets, decoupling virtual network topology from physical underlay IP addressing.Answer
  3. C
    The underlay network uses VXLAN encapsulation to map Endpoint Identifiers (EIDs) to Routing Locators (RLOCs) across physical switch interfaces.
  4. D
    The overlay network relies on OSPF longest-prefix matching to maintain physical interface state changes and hardware link adjacencies.
  5. E
    The underlay network directly inspects VXLAN Virtual Network Identifiers (VNIs) to make intermediate physical hop forwarding decisions.

Answer

The correct statements are that the underlay network provides underlying IP connectivity between fabric nodes using standard routing protocols (like OSPF or IS-IS) to reach RLOC addresses, and the overlay network uses VXLAN encapsulation to transport Layer 2/3 traffic inside UDP packets to decouple the virtual network topology from physical underlay addressing.
In software-defined network architectures such as Cisco SD-Access, the underlay consists of physical interconnects and IP routing protocols (e.g., OSPF, IS-IS) that ensure end-to-end IP reachability between fabric nodes (Routing Locators or RLOCs). The overlay runs on top of the underlay and uses VXLAN to encapsulate Layer 2 and Layer 3 traffic into UDP packets, abstracting client endpoints (EIDs) from physical topology and allowing virtual network segmentation using VNIs.

Step-by-Step Solution

1
Analyze underlay responsibilities in SDN fabric architectures.
Identified that the underlay is the physical infrastructure running standard IP routing protocols (OSPF, IS-IS, BGP) to guarantee IP reachability between fabric devices (RLOCs).
The underlay serves purely as the transport network for outer IP packet headers.
2
Analyze overlay responsibilities in SDN fabric architectures.
Identified that the overlay uses tunnel encapsulation (VXLAN in SD-Access) to transport virtual network payloads over the underlay, providing network virtualization and mobility.
The overlay abstracts user networks (EIDs) from physical switch locations (RLOCs).
3
Evaluate intermediate underlay node behavior regarding packet inspection.
Confirmed that underlay devices inspect only outer IP headers, not inner VXLAN VNI headers or overlay payloads.
Underlay switches route based on outer destination RLOC IP addresses.

Key Concept

Underlay vs. Overlay architecture separation in SDN and Cisco SD-Access fabrics
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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