This morning I have connected to the observatory using a low bandwidth connection to find further information about the nature of the problem, these are my findings...
First I have to start by explaining a bit about our network set up on site. The network connection to our building comes from the Cielo Nocturno building near by. This is the white building which can been seen on the right hand side of the Road cam images when the link is up. From this building we have a fibre link in to our observatory. The fibre link goes through a media converter, which converts it to standard Cat5 Ethernet. The Ethernet connection is then wired in to our external switch.
Our first piece of information about the nature of the failure is that our Night-Sky (Cielo Nocturno in Spanish) camera is still working. This camera is on the same subnet as our observatory, so from this we can assume that the network link to the Cielo Nocturno building is working, and that the routing for our subnet is also working.
At this point I connected to the observatory on our backup low bandwidth link.
From here I tried to ping various machines on the external network, some in our building, some outside. The results are that pinging machines outside of our building fail, but the machines inside our building succeed. This tells us that the external network link is down, but that the external switch in our building is operating normally.
From here I logged in to our external switch (which is managed) to check the state of the external up-link port. The switch confirmed that the Ethernet link to the media converter is up. This suggests that the media converter is powered, but doesn't tell us if it is actually functioning properly.
To summarise all of this information: It would appear that all of our hardware is fine. The problem is almost certainly in the link between our building and Cielo Nocturno. As a result of this I have submitted an urgent job request for the Observatorio del Teide maintenance team to investigate and resolve the issue.