Thanks all for the info on UDLD. In my case the test did not work as
expected because the port was in auto-negotiate, as it should be.
Disabling it allowed the port to stay up even if the other end was
down (no light).
Enabling the UDLD worked as I would expect in this case. But in the
end the auto should be enabled and the UDLD will detect converter
issues or patch panel issues.
The one thing I am curious about is, what happens if the other end is
not CISCO but the next hop L2 is. Does the UDLD packet (01-00-0C-CC-
CC-CC) pass thru? ( I would say yes). I would guess this could cause
strange link failures and is why UDLD is not on by default.
The best reference for UDLD is the rfc 5171
Jeff
On Jun 30, 2009, at 11:25 AM, Tim Stevenson wrote:
GE/10G can detect a physical unidirectional fiber link itself, UDLD
is not necessary to detect this type of failure.
UDLD is needed for exactly the case you mention, or for cases where
one side of the link is "braindead" but does not bring the physical
link down (ie, software problem).
HTH,
Tim
At 07:57 AM 6/30/2009, Peter Rathlev stated:
On Tue, 2009-06-30 at 09:59 -0400, Jeff Fitzwater wrote:
> We have had a few strange unidirectional link problems and I
thought
> that I could detect them using UDLD. So I thought I knew how it
> worked. I
...
> I thought that breaking on side of the fiber would only bring
down one
> end LINK since the other still thought it was connected.
>
> I then disabled the UDLD and disconnect the fiber again and still
had
> both ends show link failure.
Just tried this between a 3560 12.2(35)SE5 and a 2970 12.2(25)SEC2
with
the same symptoms as you describe; disconnecting one fiber doesn't
trigger UDLD but does give link down in both ends. This is also
contrary
to what I expected.
UDLD is useful in another case though: Media converters and EoMPLS
xconnected ports are transparent to UDLD but might not have link
poisoning enabled. With UDLD you would discover the loss of
connectivity
to the neighbor even though the link doesn't go down.
As long as the link actually goes down the UDLD isn't needed
anyway. :-)
Regards,
Peter
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Tim Stevenson, [email protected]
Routing & Switching CCIE #5561
Technical Marketing Engineer, Cisco Nexus 7000
Cisco - http://www.cisco.com
IP Phone: 408-526-6759
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