I was recently tipped off by our ISP that our router is transmitting PDUs to their switch, and told that if we don't turn it off they might be able to penetrate our LAN. What do they mean that we are transmitting PDUs? If we do turn off the PDUs transmission, won't it affect any services that the router transmits?
I think what they mean is BPDUs, or Bridge Protocol Data Units, not PDUs. PDUs are just Protocol Data Units, ie layer 2 frames or layer 3 packets. The ISP seeing BPDUs means that the interface that connects to your ISP is likely set as a trunk port, and it is transmitting more than one VLAN's frames to it. If this is an actual router it won't do this unless you've configured more than one subinterface and you are tagging the packets with those vlans, but I think it is more likely you are using a switch not a router. If that's the case you need to ensure that the interface is not set as a trunk, or dynamic (the default). Do a "show run int " to see. If you see no switchport mode statement then it is set to dynamic, if it says switchport trunk then it's a trunk. I think it's probably set to dynamic in which case you need to change it to mode access: