Cisco Support Community
Community Member

Link Detection Failure - Concept Questions


Please check the figure 27-1 in:, regarding UDLD operation:

1) In the failure event, why Switch "A" doesn?t disable its port based only in Layer 1 mechanisms included in gigabit ethernet standard? what is missing in the standard? That means port in switch "A" is always UP? Why?

2) Assuming switch "A" doesn?t disable the failed port (for some reason, no UDLD support), is there some way for Switch "B" to detect the remote failure? Then, is it port in Switch "B" always UP?

3) Do you know some group, working around the standardization of UDLD?


Re: Link Detection Failure - Concept Questions


UDLD is a layer 2 protocol, to detect unidirectional link failures.

It works together with autonegotiation.

Autonegotiation takes care of problems on layer 1.

For example, assume that from Switch A to Switch B, one pair of transmission media is not working as follows,

Switch A's transmit pair is properly connected to Switch B's Receive pair.

But Switch A's receive pair is not properly connected to Switch B's Transmit pair.

In the above scenario, when autonegotiation is enabled on this link, the ethernet link will not comeup.

In this scenario, UDLD doesn't play any role as the fault detection is done at layer 1 itself.

Assume that the pair's are properly connected, but still for some reason, data transmission is affected in one pair of the media.

In this scenario, as far as autonegotiation is concerned the link is up from layer 1 perspective.

However the data transmission is affected in one direction.

This fault condition can only be detected by UDLD as it operates in layer 2.

Please check this link for more explanation on UDLD and how it works.

As far as the explanation provided in the link you have mentioned, it is dealing with a fault condition happening at layer 2 not at layer 1.

With out UDLD working in layer 2, there is no way the fault at layer 2 can be identified.

There is no such mechanism available in the ethernet standard.

UDLD by cisco is a draft standard.

Probably soon other vendors will also look in to bundling this feature for ethernet links.

Hope this helps.


CreatePlease to create content