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Zorluk: ZorSoftware-Defined Architectures: Overlay, Underlay, and Fabric

An enterprise network implements a Cisco Software-Defined Access (SD-Access) solution where two endpoints residing on different IP subnets communicate across a campus fabric. Fabric Edge Switch 1 encapsulates packets from Endpoint A destined for Endpoint B on Fabric Edge Switch 2. Which two statements correctly describe the operational roles and behaviors of the underlay and overlay networks during this communication flow?

  1. The underlay network uses standard Layer 3 routing protocols to provide IP reachability exclusively between the Routing Locator (RLOC) addresses of the fabric edge nodes.Cevap
  2. The overlay network encapsulates original host packets inside VXLAN headers containing outer source and destination IP addresses corresponding to the fabric edge RLOCs.Cevap
  3. C
    Intermediate underlay switches inspect the inner VXLAN header of in-flight packets to perform Layer 2 MAC address forwarding for endpoint traffic.
  4. D
    All intermediate underlay routers must participate in the LISP control plane protocol to map host Endpoint Identifiers (EIDs) to destination subnets.
  5. E
    The underlay network terminates and re-encapsulates the VXLAN tunnel at each intermediate hop to enforce Layer 2 security policies.

Cevap

The correct statements are that the underlay network provides Layer 3 IP reachability strictly between fabric edge node Routing Locators (RLOCs) using standard routing protocols, and the overlay network encapsulates host frames using VXLAN with outer IP headers addressing those RLOCs.
In a Cisco SD-Access fabric, the underlay network is responsible solely for providing robust Layer 3 IP connectivity between fabric node loopback addresses, known as Routing Locators (RLOCs). The overlay network abstracts endpoint traffic by using VXLAN encapsulation at the ingress fabric edge node, wrapping the original packet inside an outer IP header directed to the egress fabric edge node's RLOC.

Adım Adım Çözüm

1
Identify the primary responsibility of the underlay network in a software-defined fabric.
The underlay consists of physical switches, routers, and Layer 3 links running an IGP (such as IS-IS or OSPF) to establish IP reachability between node loopbacks (RLOCs).
The underlay functions purely as a IP transport mechanism for outer tunnel traffic without tracking endpoint host routes or overlay VLANs.
2
Analyze how the overlay data plane handles host packet transport across the fabric.
Fabric Edge Switch 1 acts as a VXLAN Tunnel Endpoint (VTEP), wrapping the host's original Layer 2 frame/Layer 3 packet into a VXLAN header and outer UDP/IP header with source RLOC of Edge 1 and destination RLOC of Edge 2.
VXLAN encapsulation isolates endpoint addressing (EIDs) from the physical topology and enables virtualized Layer 2 and Layer 3 overlays.
3
Evaluate intermediate underlay device functions to eliminate incorrect options.
Intermediate core/distribution switches in the underlay route packets based solely on the outer IP header (destination RLOC). They do not run LISP, decapsulate VXLAN, or inspect inner MAC/IP headers.
Maintaining clear separation between underlay transport routing and overlay control/data plane operations keeps the network core simple, scalable, and stateless regarding endpoints.

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Software-Defined Architectures: Overlay, Underlay, and Fabric

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