UDLD on twisted pair ports

Unanswered Question
Apr 14th, 2010

Hi all

After some reading on UDLD, am i right in saying that it will only work if the thing plugged into the switch is udld aware? so if in aggressive mode on my cat5 links, if I plug a pc in udld will not work?? is this true??



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Giuseppe Larosa Wed, 04/14/2010 - 08:01

Hello Carl,

UDLD keeps a neighbor state machine per port : if on port X  no UDLD neighbor has ever been seen, UDLD even in aggressive mode will not be triggered (it will not perform reaction).

Conversely, if an UDLD neighbor is present on port Y with UDLD aggressive enabled and the port stops to receive UDLD messages UDLD reacts and will put the port in error disable (if in aggressive mode)

Hope to help


carl_townshend Wed, 04/14/2010 - 08:13


Thanks for the reply, so are we saying that if im only plugging normal machines in the ports there is no use enabling UDLD? as it will not work??

Giuseppe Larosa Wed, 04/14/2010 - 11:20

Hello Carl,

yes, as I said UDLD requires a cooperating neighbor that sends back the frame adding some own information if no UDLD neighbor is seen on the port UDLD will never be in use on that port.

UDLD objective is to be used on inter switch high loaded links in order to help STP in preventing bridging loops not towards PCs or servers.

Hope to help


Leo Laohoo Wed, 04/14/2010 - 15:32

In my personal opinion, don't use UDLD port aggressive on copper lines.


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