This is a very strange problem that I have. We have a site in which is connected to two other sites via BT Les 100 lines (dark fibre effectively). Last week the switches in one site just stopped any spanned vlan's working across the data links.
The links are still operational and all layer 3 trafic is running as normal - we just haven't got any layer 2 traffic.
Nothing was changed on the switches (in fact there weren't any network engineers on site) or the routers but the spanned vlans just stopped. rebooting the switches didn't make any difference either.
The switches are 3750 stack units running ios 12.1(19) which I know is out of date but they have been running quietly for the last 3 years without a problem.
Any suggestions - I am going to replace them with a 4507 in the next week or so anyway but was just curious as to the fault.