Layer 2 Switching Commands 776recognize only the sending failures on unidirectional links. If all devices in thenetwork support UDLD, this functionality is enough to detect allunidirectional links.Processing UDLD Traffic from NeighborsEvery UDLD-capable device collects information about all other UDLD-capable devices. Each device populates UDLD echo packets with collectedneighbor information to help neighbors identify unidirectional links. Everyframe basically contains the device ID of the sender and the collection ofdevice IDs of its discovered neighbors.UDLD in Normal-modeIn normal mode, a port’s state is classified as undetermined if an anomalyexists. These include the absence of its own information in received UDLDmessages or the failure to receive UDLD messages. The state ofundetermined has no effect on the operation of the port. The port is notdisabled and continues operating as it previously did. When in normal mode,a port is diagnostically disabled for the following cases:a UDLD PDU is received from partner that does not have the port’sown details (echo).b When there is a loopback, information sent out on a port is receivedback as is.UDLD in Aggressive-modeAggressive mode differs from normal UDLD mode – it can diagnosticallydisable a port if the port does not receive any UDLD echo packets after abidirectional connection was established. It expands the cases when port canbe disabled. There can be several causes for a port not to receive UDLDechoes. These include:• A link is up on one side and down on the other. This can occur on fiberports if the transmit port is unplugged on one side.• Loss of connectivity, i.e. the port is neither transmitting nor receiving, butthe port also reports it is up.
