We have a core network comprising of two Cat6k switches. For a couple of sites we have chosen to install two LAN Extention-type services terminating in a single switch (usually 2950-series). The LES circuit are provided by the telco and are media converted at both ends from dark fibre onto a UTP presentation; these then connect to our core switches.
The issue is that on a number of occasions, a fault has developed at one of these sites where one of the LES links becomes unidirectional. Since the telco's media converters do not reflect the state of the fibre port in the state of the UTP port (i.e. the fibre port can be unidirectional, or disconnected completely) and the UTP connection remains up. I believe this is causing an STP loop which then proceeds to affect the remainder of the network (HSRP instability and consequential loss of OSPF adjacencies etc.)
I cannot see a simple solution to this problem, apart from perhaps using UDLD in aggressive-mode (no non-aggressive UDLD will only detect a unidirectional link in this scenario, not actively errdisable any port). UDLD aggressive-mode appears to offer a potential solution on the bench, but has other issues. For example, if the remote switch reloads (e.g. after a power failure), then aggressive-mode on the core switch will err-disable the links because the remote end has gone down). This then requires manual intervention to fix.
