VXLAN Overhead & Efficiency Modeler
A precision simulator for VXLAN overlay fabrics. Model the exact impact of encapsulation on your MTU and total available bandwidth.
Simulation Params
Packet Overhead Analysis
For TCP traffic traversing EVPN-VXLAN, the **MSS (Maximum Segment Size)** must be reduced to account for the encapsulation. If the absolute path MTU is 1500 bytes, the VXLAN overhead (typically 50 bytes) dictates a maximum IP payload of 1450 bytes. Subtracting the internal IPv4 and TCP headers (40 bytes), the ideal MSS should be set to **1410 bytes** to prevent performance-killing ICMP "Fragmentation Needed" events.
In modern Leaf-Spine AI fabrics, implementing **Jumbo Frames (9000-9216 bytes)** on the underlay is mandatory. This provides sufficient "headroom" for nested encapsulation, multi-level VLAN tagging, and security headers while still allowing the standard 1500-byte client Ethernet frame to pass without fragmentation, significantly reducing CPU interrupts at the VTEP (Virtual Tunnel Endpoint).
1. The Anatomy of 50 Bytes: VXLAN Framing
A VXLAN packet is a "Layer 2 inside Layer 3" structure. Unlike standard VLANs which insert a tag, VXLAN wraps the entire original frame in a new set of headers.
Overhead Breakdown
Total Overhead: 50 Bytes. If the inner Ethernet frame contains a VLAN tag, the overhead effectively grows as that tag is encapsulated along with the rest of the frame.
2. The 1550 Rule: Bridging the MTU Gap
In a native IPv4 network, the MTU is 1500. If we add 50 bytes of VXLAN, the packet becomes 1550.
Fragmentation (MTU 1500)
If the underlay only supports 1500, every VXLAN packet is fragmented into two. This doubles the PPS (Packets Per Second) for the same bandwidth, crushing firewall and router CPU performance.
Clean Tunnel (MTU 1550+)
By increasing the underlay MTU to 1550 or 9000 (Jumbo), we ensure zero fragmentation. The inner packet remains 1500, and standard host stacks require no modification.
3. VNI Scalability: Beyond the 4096 Limit
The primary driver for VXLAN wasn't encapsulation, but the exhaustion of VLAN IDs.
24-Bit Identifier
1. **VLAN**: 12 bits = 4,096 segments. (Legacy Enterprise)
2. **VXLAN VNI**: 24 bits = 16,777,216 segments. (Hyperscale Cloud)
3. **Entropy**: The source UDP port is hashed from the inner L2/L3/L4 headers, enabling perfect ECMP (Equal-Cost Multi-Path) spreading without the switch needing to inspect the inner packet.
4. VTEP Forensics: Hardware vs. Software Endpoints
A **VTEP** is where the VXLAN magic happens. It can be a software switch (Linux kernel) or a hardware ASIC (Arista/Cisco).
Frequently Asked Questions
Technical Standards & References
Related Engineering Resources
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