I have found that -5dB or -10dB attenuators must be used on the send or receive strands between Cisco LX connected switches at relatively short distances of < 1 km over standard singlemode fiber. Other Vendors' SFPs rated up to 25 km do not need attenuators at distances <1 km. -----Original Message----- From: Jon Heise [mailto:jon@smugmug.com] Sent: Thursday, January 26, 2012 9:41 AM To: Pierre-Yves Maunier Cc: nanog@nanog.org Subject: Re: LX sfp minimum range Awesome, i got some single mode LC LC fiber off monoprice, sounds like i should be all set for this. Thanks for everyones info - Jon On Thu, Jan 26, 2012 at 6:24 AM, Pierre-Yves Maunier <nanog@maunier.org>wrote:
2012/1/26 David Storandt <dstorandt@teljet.com>
You can put a 3dB or 5dB optical pad on the link if the receiver can't handle zero-distance optical power.
We're using SFP LX for a couple of years even in back to back configuration for equipments within the same rack with a 1 meter patch cord without any problem.
Max TX is -3dBm, Max RX sensivity is -3dBm so there is no problem.
1. I don't think I've ever had a LX SFP that TX at -3 dBm, they're usually around -5 to -7 dBm.
and example in a live router :
pymaunier@re1.tcr1.rb.par> show interfaces diagnostics optics ge-7/3/* | match "Laser output power " Laser output power : 0.3160 mW / -5.00 dBm Laser output power : 0.1800 mW / -7.45 dBm Laser output power : 0.2600 mW / -5.85 dBm Laser output power : 0.3210 mW / -4.93 dBm Laser output power : 0.3070 mW / -5.13 dBm Laser output power : 0.3200 mW / -4.95 dBm Laser output power : 0.3180 mW / -4.98 dBm Laser output power : 0.3140 mW / -5.03 dBm
2. You can assume a patch cord add between 0.2 to 0.5 dB of attenuation so even with a SFP TX at -3dBm, you won't receive at the Max RX sensitivity.
-- Pierre-Yves Maunier
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