The custom 14789 was picked to keep our demux socket off the ports other
VXLAN software listens on, but a private default surprises operators and
blocks interop with gear that pins the standard port. Default to the
IANA-assigned 4789 instead; the gns3_server.conf override is unchanged
for computes where swarm overlay/flannel-style VXLAN already owns 4789.
Mixed-version deployments must pin the port on every compute until they
all run the new default, or the two tunnel ends will disagree.
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