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

In a software-defined fabric deployment, an engineer is preparing the physical network switches prior to building virtual tunnels. Which essential service must the underlay network provide to enable overlay operational functionality?

  1. A
    Encapsulation of end-user data frames into VXLAN tunnel packets
  2. Reliable Layer 3 IP reachability between all physical fabric devicesAnswer
  3. C
    Centralized database lookup services for endpoint IP-to-location mapping
  4. D
    Parsing REST API payloads to push JSON configuration files to switches

Answer

Reliable Layer 3 IP reachability between all physical fabric devices
The physical underlay network acts as the foundational transport layer in software-defined architectures. Its mandatory function is to establish end-to-end Layer 3 IP reachability between physical fabric nodes (such as switches and VTEPs) using traditional routing protocols like OSPF, IS-IS, or BGP. Overlay tunnels rely entirely on this underlying IP reachability to transmit encapsulated packets.

Step-by-Step Solution

1
Identify the role of the physical underlay in software-defined network architectures.
The underlay consists of physical switches, routers, links, and Interior Gateway Protocols (such as OSPF or IS-IS).
The main purpose of the underlay is providing standard Layer 3 IP packet transport across physical devices.
2
Determine the dependency of the overlay on the underlay.
Overlay virtual tunnels (e.g., VXLAN tunnels between VTEPs) transport virtualized traffic over the physical network.
Overlay tunnels cannot be established or forward traffic unless the underlay provides IP reachability between the physical IP addresses of the tunnel endpoints.

Key Concept

Underlay Layer 3 IP Reachability Requirement
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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