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Verified against the running kernel's sources (7.2.6): the 50-byte headroom is 20 outer IPv4 + 8 UDP + 8 VXLAN + 14 inner Ethernet header (vxlan_headroom(), include/net/vxlan.h); the vxlan driver has no NETDEV_CHANGEMTU handler so a live tunnel never follows its lower device, while the bridge does follow its ports at runtime (NETDEV_CHANGEMTU -> br_mtu_auto_adjust). Document the operational consequence: the wiring path re-creates the device on every node start and link (re)creation, so raising the underlay MTU on both computes and stop/start-ing the nodes is the fix — no manual ip link needed.