1G speed on 10G transceiver

Hey guys,

just a small story I wanted to share with you.

Yesterday we had a scheduled downtime at a customer for a core switch upgrade and I ran into a problem I never knew even existed.

We have a modular HPE Comware 10508 as core switch, which was running a 2014 firmware. I checked the release notes for incompatibilities with certain modules or transceivers but did not find any problems with the new firmware. Everything worked fine with the upgrade to the current version, management modules upgraded, then the switching fabrics, then the line cards. All came up normal, links came up, the network was back online.

I was ready to celebrate an upgrade without problems when we noticed that our darkfibre connection to the datacenter was down. The interface was down.
I checked the transceiver, HPE 10G LR SFP+, ok.
I checked the received dBm on the transceiver, looks fine, the link should come up.
I changed the config to another module but it still didn't work.

Then the customer also compared the config to the old config file and noticed that the interface was set to "speed 1000" in the old config. I was very confused at that point, as I thought you can not change the speed of a transceiver to a lower speed. And the switch confirmes this, as the command is not allowed in the new firmware ("This operation is not supported").

We used a 1G LX transceiver and the link came up immediately.

Have you guys ever seen that a transceiver is set to a lower speed? I never knew this is even possible and I can't find any documentation on it. Sure you can use a 1G transceiver in an SFP+ port, but setting a 10G transceiver to 1G?