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

A network operations engineer is validating traffic transport across an enterprise Cisco Software-Defined Access (SD-Access) campus environment. The underlying physical switches use IS-IS routing to maintain full IP reachability between switch loopback interfaces. Which statement accurately describes how the underlay and overlay components interact during frame forwarding between endpoints?

  1. The underlay network provides basic IP reachability between switch loopback interfaces (RLOCs), while the overlay network uses VXLAN encapsulation to tunnel endpoint frames across the underlay.Answer
  2. B
    The overlay network operates standard OSPF routing across physical interfaces, while the underlay network encapsulates endpoint traffic into LISP packets to bypass Layer 3 routing.
  3. C
    The underlay network manages virtual MAC address tables using VXLAN encapsulation, while the overlay network maintains physical link status and port security profiles.
  4. D
    The overlay network provides physical copper/fiber links between fabric nodes, while the underlay network creates logical BGP tunnels to transport local broadcast domains.

Answer

The underlay network provides basic IP reachability between switch loopback interfaces (RLOCs), while the overlay network uses VXLAN encapsulation to tunnel endpoint frames across the underlay.
In software-defined network (SDN) architectures such as Cisco SD-Access, the underlay consists of physical hardware devices interconnected via robust Layer 3 routing protocols (OSPF, IS-IS, BGP) that guarantee IP connectivity between Routing Locators (RLOCs/VTEPs). The overlay operates logically above this underlay, encapsulating endpoint data frames into VXLAN headers to deliver virtualized networks across the physical infrastructure.

Step-by-Step Solution

1
Identify the primary function of the underlay network in a software-defined fabric.
The underlay consists of physical switches, routers, and IP routing protocols (such as OSPF or IS-IS) establishing unicast IP reachability between fabric nodes.
Underlay routing provides the underlying transport infrastructure needed for tunnel end-points (VTEPs / RLOCs) to communicate.
2
Identify the primary function of the overlay network.
The overlay creates logical tunnels over the underlay using encapsulation protocols such as VXLAN.
Encapsulating endpoint traffic inside UDP/IP packets allows Layer 2 and Layer 3 virtual networks to run independently over the physical underlay infrastructure.

Key Concept

Underlay vs Overlay Separation in Software-Defined Fabrics
Estimated Time:1m 15s
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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