5. Fragmentation and MTU

5. Fragmentation and MTU

Geneve encapsulation adds overhead to the original packet. This includes the Outer Ethernet, IP, UDP, and Geneve headers. This overhead reduces the effective Maximum Transmission Unit (MTU) available to the tenant.

To avoid fragmentation and the associated performance penalty, the underlay network SHOULD support a large enough MTU to accommodate the encapsulated packets. Ideally, the underlay MTU should be at least (Tenant MTU + Geneve Overhead + Outer Headers). Commonly, jumbo frames (e.g., 9000 bytes) are used in the data center underlay to handle the encapsulation overhead for standard 1500-byte tenant frames.

If the underlay MTU is insufficient, fragmentation will occur. Fragmentation can happen at two levels:

1. Inner Fragmentation: The tenant system fragments the packet before encapsulation because it exceeds the Tenant MTU.
2. Outer Fragmentation: The tunnel endpoint or a transit device fragments the encapsulated packet because it exceeds the Underlay MTU.

Outer fragmentation is generally discouraged because reassembly at the tunnel endpoint can be resource-intensive. Techniques such as Path MTU Discovery (PMTUD) can be used to determine the appropriate MTU.