Unanswered Question
Feb 2nd, 2009


In a service provide environment, when udld is deployed on spanning-tree bpdu filtering enabled port which is connected to customer end, will it help detect STP loop or perform any other function?

Thanks in advance

I have this problem too.
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didyap Fri, 02/06/2009 - 11:38

Loop guard and Unidirectional Link Detection (UDLD) functionality overlap, partly in the sense that both protect against STP failures caused by unidirectional links. However, these two features differ in functionality and how they approach the problem.

Based on the various design considerations, you can choose either UDLD or the loop guard feature. In regards to STP, the most noticeable difference between the two features is the absence of protection in UDLD against STP failures caused by problems in software. As a result, the designated switch does not send BPDUs. However, this type of failure is (by an order of magnitude) more rare than failures caused by unidirectional links. In return, UDLD might be more flexible in the case of unidirectional links on EtherChannel. In this case, UDLD disables only failed links, and the channel should remain functional with the links that remain. In such a failure, the loop guard puts it into loop-inconsistent state in order to block the whole channel.

Giuseppe Larosa Mon, 02/09/2009 - 12:41

Hello Kai,

if you have enabled spanning-tree bpdu filtering on the port is because the service provider wants to avoid to take part in customer's STP topology.

So UDLD can help to detect unidirectional condition on the link.

If using bpdu filtering the topology has a single point of contact between SP and customer otherwise a loop could form.

To be noted that in a context that uses RSTP 802.1W either in RPVST or in MST 802.1s UDLD is too slow in comparison with RSTP convergence capabilities.

Hope to help



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