[c-nsp] Routing design question

Tassos Chatzithomaoglou achatz at forthnet.gr
Mon Oct 15 05:48:50 EDT 2007

Hi Lincoln,

If i remember right, there are some SFPs that use a single fiber for both tx/rx.

Does UDLD help in such cases? Is there a possibility for a problem in only one direction?


Lincoln Dale (ltd) wrote on 12/10/2007 5:39 πμ:
>>> Will auto-neg signal one-way fiber failures (after the link has already
>>> been brought up and autoneg'ed successfully)?  Never tried that.
>> Yes, it works much better (and faster) than UDLD. The endpoint which
>> has lost receive fiber will immediately signal "RemoteFault" to the
>> other end and line protocol will go down on both sides within miliseconds.
> the primary intent of UDLD is detection of mis-cabling at layer-2.
> e.g. lets say you had three devices, cabled with tx/rx in a triangle.
> the link may well come up at layer-1 but it sure will do bad bad things at layer-2 particularly with protocols like spanning-tree!
> UDLD will detect that, autoneg won't.
> the recommendation would be to use both autoneg (layer-1) and UDLD (layer-2).
> cheers,
> lincoln.
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