Both ends are back on autoneg at 1000 Mbps and stable; UDM SFP+2 restored to
autoneg True / setting_preference auto from the payload staged before the
original change. ~50 s of renegotiation, nothing rebooted.
The force failed progressively and I called it too early. Errors on the
ESH-Media side went 200 at link-up, 221 at 42 minutes, 416 by four hours, with
flapping the operator noticed before my soak had accumulated enough to raise
it. I read a two-minute flat window as a plateau and said so; a marginal link
declares itself over hours, not minutes.
Do not re-force this port. The cable is being replaced at the weekend and a
correctly-coded DAC negotiates 10G on its own.
Operator is keeping the override rather than reverting: he has a replacement
cable and will run it at the weekend. Recorded as a held position so a later
session does not revert it as an oversight or re-run the diagnosis.
Also corrects a claim this runbook made an hour earlier. I wrote that the
zero-error 1G link ruled out a marginal cable. It did not — it only proved the
cable was clean at 1G. The two-hour soak shows errors climbing monotonically at
10G (200 to 221 rx, ~1 per 23 MB) plus one self-recovering link drop, so the
autoneg fallback had a real basis and the fix is the cable, not the config.
The OEM SFP-H10GB-CU3M between ESH-Media SFP+1 and UDM SFP+2 linked cleanly
at 1000 with zero errors on both ends, which rules out a damaged cable — a
marginal one links at 10G and throws CRCs rather than dropping a rate. The
switch reported sfp_compliance "Unknown": it reads the EEPROM but cannot
parse the compliance codes on a third-party cable wearing Cisco coding, so
autoneg falls back to the safe rate.
A TP-Link TL-SM5220 DAC on the adjacent UDM port runs at 10000 with the same
speed_caps and autoneg on, which exonerated the port, the firmware and
autoneg before anything was changed.
Forced autoneg off / 10000 / full duplex on the UDM end only. ESH-Media's
end followed to 10000 on its own, which is the proof the cable was always
capable and only the negotiation was wrong. Both ends now 10000.
The UDM end was chosen because it is the recoverable one: ESH-Media reaches
the controller through this link, so a failed force there would strand the
switch. The path to the UDM was traceroute-verified not to cross the link,
and the revert payload was written before the forward one was sent.
Cost ~60 s of link renegotiation. ESH-Media, the E7-ESH-Media AP and the
USW Flex XG ESH-Office-Vuong all returned at uplink_speed 10000 with uptimes
unbroken; the Zigbee coordinator and HA's path to it were re-verified.
⚠ The port is now pinned: a non-10G module will not link there. Documented,
along with the ~200 link-up errors that then went flat and the fact that
steadily climbing errors would mean the cable genuinely cannot do 10G.
vh