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

An enterprise network administrator deploys a Cisco SD-Access fabric overlay using VXLAN for data plane encapsulation over an OSPF-based Layer 3 routed underlay network. During validation, control plane communication between the LISP Control Plane Node and Fabric Edge switches operates normally, and all underlay loopback interfaces show full ping reachability. However, host endpoints in different subnets attached to separate Fabric Edge switches fail to communicate, with packet captures revealing that encapsulated data packets are dropped by transit routers along the underlay path. Which underlay infrastructure condition is causing this traffic delivery failure?

  1. The Maximum Transmission Unit (MTU) on intermediate underlay router interfaces is set to 1500 bytes, causing VXLAN encapsulated frames with DF flags set to be dropped.Answer
  2. B
    The underlay OSPF routing process is missing redistributions for host Endpoint ID (EID) subnets into the transit routing table.
  3. C
    The host-facing access switchports on the Fabric Edge switches are configured with static access VLANs rather than 802.1Q trunks.
  4. D
    The underlay transit routers are unable to build Layer 2 MAC address tables for the host endpoints.

Answer

The MTU on intermediate underlay router interfaces is configured to the standard 1500-byte limit without accounting for VXLAN encapsulation overhead, leading to packet drops along the routed transit path.
In software-defined fabric architectures using VXLAN encapsulation (such as Cisco SD-Access), outer headers add at least 50 bytes of overhead to every packet transmitted between Fabric Edge nodes. Because VXLAN packets typically set the Don't Fragment (DF) bit in the outer IP header, intermediate underlay routers with a standard 1500-byte MTU cannot forward packets exceeding 1500 bytes and will drop them. Therefore, the underlay network interfaces must be configured with an elevated MTU (e.g., 9000 bytes for jumbo frames or at least 1550 bytes) to accommodate the overlay encapsulation.

Step-by-Step Solution

1
Analyze the role of the underlay and overlay components in a fabric architecture.
The underlay provides Layer 3 IP reachability between Fabric Edge loopback interfaces (RLOCs). The overlay uses VXLAN encapsulation to tunnel Layer 2/Layer 3 payload between endpoints.
Understanding the separation of underlay transport and overlay encapsulation isolates where packet processing issues occur.
2
Calculate the header overhead introduced by VXLAN encapsulation.
VXLAN encapsulation adds an outer UDP header (8 bytes), VXLAN header (8 bytes), outer IP header (20 bytes), and outer Ethernet header (14 bytes), totaling 50 bytes of additional encapsulation header.
A standard 1500-byte IP packet becomes 1550 bytes after VXLAN encapsulation.
3
Determine the impact of underlay interface MTU on encapsulated packets with Don't Fragment (DF) bits set.
If transit underlay interfaces retain the default 1500-byte MTU, any 1500-byte host payload encapsulated in a 1550-byte VXLAN packet will exceed the MTU and be dropped by underlay routers.
To support VXLAN fabric overlay traffic, all underlay routed links and switch interfaces must have jumbo frames or an increased MTU (minimum 1550–1600 bytes, typically 9000 bytes) configured.

Key Concept

Underlay MTU Requirements for VXLAN Overlay Encapsulation
Estimated Time:2m 0s
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Topics

Software-Defined Architectures: Overlay, Underlay, and Fabric

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