Cisco SD-Access Logical Networks: Overlay vs Underlay

350-401 By islever Editorial Last updated

Explain the working principles of the Cisco SD-Access solution

Short answer

D The overlay network provides Layer 2 and Layer 3 logical networks in the Cisco SD-Access architecture.

Question

Which technology is used to provide Layer 2 and Layer 3 logical networks in the Cisco SD-Access architecture?
  1. underlay network
  2. VPN routing/forwarding
  3. easy virtual network
  4. overlay network Correct Answer

Key Takeaways

The exam tests your ability to map 'logical' network functions to the 'overlay' component, while ensuring you do not confuse it with the 'underlay' which handles physical connectivity.

This question tests the fundamental architecture of Cisco SD-Access, specifically distinguishing between physical transport and logical network services. The page establishes that the overlay network is the technology providing Layer 2 and Layer 3 logical networks.

Common mistake: Candidates often select 'underlay network' (A) because they focus on the physical infrastructure rather than the virtualized logical service layer described in the prompt.

Expert Analysis

Why the Answer Is Correct

In Cisco SD-Access architecture, the solution is divided into an underlay and an overlay. The underlay consists of the physical IP-connected devices that provide basic connectivity. The overlay network (Option D) is built on top of the underlay and provides the logical Layer 2 and Layer 3 networks (VNIs/VNIs) that are decoupled from the physical topology. This separation allows for flexible policy enforcement and mobility independent of the physical location.

Why the Other Options Are Wrong

Underlay network (A) refers to the physical IP fabric (routers/switches) that transports the overlay traffic; it does not create the logical tenant networks itself. Easy Virtual Network (C) is a legacy feature for simplifying L2/L3 integration but is not the core technology defining SD-Access logical segmentation. VPN routing/forwarding (B) is a generic MPLS or VRF concept, whereas SD-Access uses VXLAN-based overlay encapsulation for its logical networks.

Community Comment Notes

Community consensus strongly supports Option D. As noted by user techriese, "D is correct pysical = underlay logical = overlay." Another commenter, AbdullahMohammad251, reinforces this definition: "An overlay network is a logical or virtual network built over a physical transport network called an underlay network." Some users like Rojo94 argue about semantics, noting "An overlay network is not a technology," but in the context of Cisco certification, the overlay plane is the designated answer for logical network provision.

Exam Strategy

Always look for keywords like 'logical', 'virtual', or 'tenant' when asked about SD-Access services; these point directly to the Overlay. If the question mentions 'physical', 'IP connectivity', or 'transport', choose the Underlay.

Frequently Asked Questions

What is the difference between SD-Access underlay and overlay?

The underlay is the physical IP network providing connectivity. The overlay creates the logical virtual networks (VXLANs) for tenants.

Does SD-Access use VRFs for logical networks?

While VRFs can be used, SD-Access primarily uses VNIs (VXLAN Network Identifiers) within the overlay to segment logical networks.

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Discussion 11 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 11

  1. techriese 👍 8 Answered D
    D is correct pysical = underlay logical = overlay
  2. youtri 👍 3
    correct ,he said logicall network,if it was physical network it would be underlay network
  3. AbdullahMohammad251 👍 2 Answered D
    An overlay network is a logical or virtual network built over a physical transport network called an underlay network.
  4. schattencr 👍 2
    Yes it is
  5. timtgh 👍 1
    Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for route cache maintenance that is associated with switching paths, such as fast switching and optimum switching. https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/44sg/configuration/guide/Wrapper-44SG/cef.pdf Page 3
Show 6 more comments
  1. Rojo94 👍 1 Answered B
    This question is weird. An overlay network is not a technology. Its a concept or definition, much like layer 3 is a layer and not a technology
  2. flash007 👍 1
    logical network will be the overlay and physical network would be the underlying devices under the sd-wan fabric
  3. andy_doesnt_like_uucp 👍 1
    i'd agree C would be a most common definition
  4. andy_doesnt_like_uucp 👍 1
    D - L2&L3VNs
  5. cloud29 👍 1 Answered D
    D is correct
  6. Hack4 👍 1
    I agree

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