I have a situation which is a little different than what I've seen posted here regarding UDLD. I'm using MX100s in active/passive connecting to a stack of (3) MS225 switches. I'm using the switches to do isolated VLANs for WAN connectivity. The primary internet connections are VLAN 3001 and the cellular (MG21E) connections are VLAN 3002. Port Internet 1 on both MX100s and the ISP router are connected to port 48 on all three switches, and port LAN 2 (configured as a WAN interface) on both MX100s and the MG21E are connected to port 46 on all three switches. I have RSTP enabled and BPDU Guard set on the ports. UDLD is set to Alert Only. This is a standard configuration for several branches and I have not had any problems with it until now.
I'm bringing up another site exactly as described but what I'm seeing is switch 2 port 46, which connects to the Spare MX on LAN 2, is blocking packets. The port has an orange circle/line, The Configuration section shows "Blocking - BPDU guard activated" and the status section shows "Unidirectional link (outbound fault) (neighbor ID: 683A1EB7D106, port: 45)". I've tried disabling RSTP and BPDU Guard just to see what happens but the same results, orange circle/line and the UDLD error. I have site-to-site VPNs to several sites and when I check the status it indicates that it cannot connect tunnels on all links, which tells me port 2/46 really is blocking traffic.
I don't have time to spend lengthy calls with support so I'm hoping someone has seen this before and can offer guidance. Thanks in advance.