why udld on direct fiber connections? port just goes down...

[Sebastian Stigler]

Hello,

there are many posts about udld but still I don´t understand how this is working. I know that it should protect against L2-loops. According to the cisco documentation udld err-disable or alert (aggressive or normal mode) when a connection just sending or just receiving packets AND the link is still up...but in which scenario this happening? (perhaps defect gbic?) We try just to disconnect one fiber, the port goes down but no udld events appear. How could we test if udld is working correctly on direct fiber connections (99% direct connections in our network structure, 1% with copper-fiber media converter in between)  Makes this sense in this LAN environment? Loop-Guard is activated.

Thanks,

Sebastian

[Peter Paluch · Accepted solution]

Hi Sebastian,

On copper cablings, achieving a unidirectional link is actually almost impossible (at least I have not found a way how to cleverly damage a cable so that both ports are up but the data can be carried only in one direction). On fiber, that is another story. Older interfaces had no way of informing our side that the other side does not hear us. On those interfaces, UDLD was a must. Newer fiber interfaces usually support the so-called FEFI indication (Far End Fault Indication) by which the far end of a fiber link replaces the IDLE symbol sent during quiescent periods with another symbol signifying that it cannot hear our IDLE symbols. As a result, the port will be brought down.

While activating the UDLD on these newer interfaces may not seem as advantageous, it still provides you with protection against miswiring - split fibers and shared segments being the most prominent.

Best regards,

Peter

[Sebastian Stigler]

Thank you very much Peter, you helped me a lot.

Best Regards,

Sebastian