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Difficulty: Very hardSoftware-Defined Architectures: Overlay, Underlay, and Fabric

An enterprise network implements a Cisco Software-Defined Access (SD-Access) fabric architecture across a routed Layer 3 campus backbone running OSPF. Host A (10.1.10.15) attached to Fabric Edge 1 initiates unicast communication with Host B (10.1.10.22) attached to Fabric Edge 2 across the fabric overlay. Which statement correctly describes the operation and interaction of the underlay and overlay network components during packet transmission?

  1. A
    The underlay network inspects the inner Ethernet frame payload to build dynamic MAC address forwarding tables across intermediate underlay switches.
  2. Fabric Edge 1 queries the LISP Control Plane Node to resolve Host B's location, then encapsulates the frame in VXLAN using its own RLOC as the outer source IP and Fabric Edge 2's RLOC as the outer destination IP for underlay routing.Answer
  3. C
    Fabric Edge 1 uses OSPF longest prefix matching to directly locate Host B's /32 host route in the underlay routing table without consulting a control plane mapping database.
  4. D
    An centralized automation agent must statically push real-time MAC-to-IP forwarding entries to intermediate underlay devices via SSH before the frame can traverse the underlay.

Answer

Fabric Edge 1 queries the LISP Control Plane Node to resolve Host B's location, then encapsulates the frame in VXLAN using its own RLOC as the outer source IP and Fabric Edge 2's RLOC as the outer destination IP for underlay routing.
Software-Defined Access architectures separate control plane and data plane functions across underlay and overlay networks. LISP acts as the control plane to map Endpoint Identifiers (EIDs) to Routing Locators (RLOCs). VXLAN acts as the data plane encapsulation mechanism, wrapping the original inner Layer 2 frame inside an outer IP packet addressed from the source RLOC to the destination RLOC. The underlay network uses standard Layer 3 IP routing (such as OSPF) to transport the outer VXLAN packet across the physical topology.

Step-by-Step Solution

1
Identify the role of the fabric Control Plane (LISP)
Fabric Edge 1 queries the LISP Control Plane Node (Map-Server/Map-Resolver) to map the destination Endpoint Identifier (EID, Host B IP/MAC) to its corresponding Routing Locator (RLOC, Fabric Edge 2 IP).
The fabric overlay relies on control plane mapping to locate endpoints dynamically across the fabric.
2
Identify the role of the fabric Data Plane (VXLAN)
Fabric Edge 1 encapsulates the original Ethernet frame into a VXLAN packet, adding an outer UDP header and outer IP header containing source RLOC (Fabric Edge 1) and destination RLOC (Fabric Edge 2).
VXLAN provides Layer 2 over Layer 3 encapsulation, allowing overlay subnet extensions over an IP underlay.
3
Identify the role of the Underlay Network (IP/OSPF)
Intermediate underlay switches forward the packet strictly using standard Layer 3 destination IP routing based on the outer IP header (Fabric Edge 2's RLOC).
The underlay network's sole responsibility is providing unicast/multicast IP reachability between RLOCs.

Key Concept

SDN Fabric Architecture: LISP Control Plane, VXLAN Overlay Data Plane, and IP Underlay Separation
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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