I have 4 core 6500's that are connected in this fashion. A to B, B to C, C to D, and D to A. These core switches feed distribution switches in various wiring closets. Recently, I lost all connectivity to distribution switches off 6500 C intermittently during about a 7 hour interval. Finally at the 7th hour the connection on core 6500 A to core 6500 D logged a UDLD error and shut down with err-disabled. Since then all is stable. I theorize that during the 7 hour interval the connection was having problems, causing periodic spanning tree recalculation and temporary loss of connectivity. Now, how can I verify that there is indeed an issue on the link between core switch A and D? If there was periodic spanning tree recalculation, Why? Shouldn't UDLD have shut down the port earlier, preventing the possibility of periodic connectivity. If I shut/no shut the port UDLD err-disabled I am afraid that it might work for now, but cause the same problem again in the future. Is it possible there is no physical link problem but something else going on? Any opinions are appreciated. Thanks.