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New Member

question about udld

Reading the udld doc to try to understand how UDLD works. Attached is the a diagram which indicates that how udld happens. Knowing that UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link. Question I have is: if layer 1 of this fiber is working which means the tx and rx is properly connected between the two ports of two switches. If B able to receive packet from A on port 1, how come A will not receive packet on the connected port, will the tx and rx on different physical port? Thanks

Cisco Employee

Re: question about udld


There's been a misunderstanding.The figure depicts a single connection on 1 port. It illustrates a cross connection to show that tx and rx are cross connected.

Let me know if you have further queries.

New Member

Re: question about udld

Thanks for your reply. But I am still confusing. If this is a single port, then the connection is fine. As we know in fiber connection it always rx goes to tx, and tx goes to rx. So there is no wrong. why would possibly B able to receive from A, but A can'treceive from B?

Cisco Employee

Re: question about udld

We are talking in terms of UDLD packets. If A can get udld updates from B, but B cant recieve it from A. It might be due to interface errors, collissions, packet drops, buffer failures on either port on either switch. It can be output/input buffer failure on sending/recieving port.

The goal of the UDLD protocol is to detect whenever a bidirectional link is either broken, such that packets are sent only from one device to an other, or connected wrongly, such that the fibres are connected to different ports. Meaning, its still a single connection, but b/w two ports of fiber connector.

For each device and for each port, a UDLD packet is sent to the port it links to. The packet contains information on who sends (device and port), and whom it's supposed to be sent to (device and port). Each port checks that the UDLD packets it receives, contain the identifiers of his own device and port.

For more info:

Please post as applicable.

New Member

Re: question about udld

Thanks very much. This is very good answer to me as well as the link.

Just double check with you, it seems normal mode doesn't do any action if detects udld aging out according to this. So what does normal help?


In normal mode, if the link state of the port was determined to be bi-directional and the UDLD information times out, no action will be taken by UDLD. The port state for UDLD will be marked as undetermined. The port will behave according to its STP state.



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