Cisco SD-Access Logical Networks: Overlay vs Underlay
Which technology is used to provide Layer 2 and Layer 3 logical networks in the Cisco SD-Access architecture?
Community Insight
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.
Candidates often select 'underlay network' (A) because they focus on the physical infrastructure rather than the virtualized logical service layer described in the prompt.
Community Discussion (12 comments)
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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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