Layer 2 Switching Commands 775UDLD CommandsDell EMC Networking N1100-ON/N1500/N2000/N2100-ON/N3000/N3100-ON/N4000 Series SwitchesThe UDLD feature detects unidirectional links on physical ports. Aunidirectional link is a forwarding anomaly in a Layer 2 communicationchannel in which a bi-directional link stops passing traffic in one direction.UDLD must be enabled on the both sides of the link in order to detect aunidirectional link. The UDLD protocol operates by exchanging packetscontaining information about neighboring devices.UDLD enabled devices send announcements to the multicast destinationaddress 01-00-0c-cc-cc-cc. UDLD packets are transmitted using SNAPencapsulation, with OUI value 0x00000c (Cisco) and protocol ID 0x0111.UDLD is supported on individual physical ports that are members of portchannel interface. If any of the aggregated links becomes unidirectional,UDLD detects it and disables the individual link, but not the entire portchannel. This improves fault tolerance of port-channel.UDLD PDUs act as network control packets. They are unaffected bySpanning Tree state. Thus, they are transmitted and received regardless ofSpanning Tree state.For the successful operation of UDLD, it is required that its neighbors areUDLD-capable and UDLD is enabled on the corresponding ports. All portsshould also be configured to use the same mode of UDLD, either normal oraggressive mode.Detecting Unidirectional Links on a Device PortA device detects unidirectional links on its port via UDLD. Every UDLD-capable device distributes service information over the network via a layer 2broadcast frame. This service frame contains information about sender(source device) and all discovered neighbors. Every sender expects to receivean UDLD echo frame. If an echo frame is received, but does not containinformation about the sender itself, it implies that the sender's frames havenot reached the neighbors. This can happen when the link is able to receivetraffic but cannot send traffic. In other words, a UDLD-capable device can
