[c-nsp] UDLD vs Auto-neg

[Tim Durack]

Okay, now it makes sense:

"A unidirectional link occurs whenever traffic transmitted by the
local device over a link is received by the neighbor but traffic
transmitted from the neighbor is not received by the local device. If
one of the fiber strands in a pair is disconnected, as long as
autonegotiation is active, the link does not stay up. In this case,
the logical link is undetermined, and UDLD does not take any action.
If both fibers are working normally at Layer 1, then UDLD at Layer 2
determines whether those fibers are connected correctly and whether
traffic is flowing bidirectionally between the correct neighbors. This
check cannot be performed by autonegotiation, because autonegotiation
operates at Layer 1."



On 8/22/07, Afsheen Bigdeli <afsheenb at gravityplaysfavorites.net> wrote:
>  From the documentation:
>
> "At Layer 1, autonegotiation takes care of physical signaling and fault
> detection. UDLD performs tasks that autonegotiation cannot perform, such
> as detecting the identities of neighbors and shutting down misconnected
> interfaces. When you enable both autonegotiation and UDLD, the Layer 1
> and Layer 2 detections work together to prevent physical and logical
> unidirectional connections and the malfunctioning of other protocols."
>
> See the 'Configuring UDLD' section of any recent software config guide
> for more info.
>
> --afsheenb
>
> Tim Durack wrote:
> > A question that's been bugging me for a while: what does UDLD give me
> > that running auto-neg on both sides of a link doesn't? If I run auto,
> > link drops if the pathway goes one-way, and won't renegotiate until
> > the pathway is re-established. Isn't that all UDLD does?
> >
> > Perhaps there are some failure modes I'm not considering. Or maybe it
> > has more to do with not running auto on infrastructure links.
> >
> > Comments would be appreciated!
> >
> > Tim:>
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