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I have a strand of dark fiber between two buildings, with the run being roughly 1.5 miles/2.4km, and it is currently operating with 1Gb BX 10km optics, but when I try to put in 10Gb BX 10km optics, I don't get link. I have tried two separate pairs of the 10Gb optics that were fresh out of the box. I know both switches are capable of 10Gb, and both are in auto negotiate. Is there anything in the fiber run itself that may stop it from doing 10Gb but allow 1Gb? The 10Gb optics I'm using are FiberStore Generic SFP-10G-BX 10KM 1270/1330 (fs.com/products/74681.html and fs.com/products/74682.html).

EDIT: One switch is a Cisco 9396PX with port config as below:

interface Ethernet1/9
switchport mode trunk
switchport trunk
allowed vlan 98,899

The other switch is a Ubiquiti Edgeswitch 16XG with port config below:

interface  0/1
vlan participation exclude 1
vlan participation include 98,899
vlan tagging 98,899
exit
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    What's the make/model of your routers or switches? Most will have a CLI command you can use to find out if you have enough signal strength coming in from the remote side, e.g. show int transceiver detail ... or show int diag optics .... This information will help figure out what's going on. Dec 16 '20 at 19:51
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Dec 31 '20 at 19:40
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  1. Try to link the pair of transceivers with a simple patch cable - not all of them are compatible and the wavelengths need to match (crossed over). There's no offical standard for 10GBASE-BR (as it should be called due to using -R PCS code), so there are some degrees of freedom. (The fs.com SFPs you've linked to should work together, obviously, but possibly one's DOA.)
  2. Double check whether the SFP+ modules are compatible with your switch. Many switches only accept "original" modules or ones pretending to be. You might need to get vendor-compatible versions.
  3. Check the switch logs for error or incompatibility messages.
  4. Check the transceiver diagnostics (see Jeff's comment) - it should show at least some receive power when plugged together.
  5. With a low-power signal or significant error counts on the ports, check the fiber for overbending (< 15 mm radius) and the terminals for dirt or damage (requires a scope). It could also be a faulty splice.

On marginal fiber it's theoretically possible for 10G to fail while 1G barely works but that's very unlikely, especially on such a short run.

A wavelength problem can be ruled out pretty much: 1000BASE-BX10 uses 1310 and 1490 nm while (most) 10GBASE-BR transceivers use 1270/1330 nm - unless of course, there's a WDM multiplexer somewhere, splitting off 1270 nm.

My guess is that it's #2.

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  • I don't think the SFP+ module compatibility is an issue because the current 1GB SFPs are also Fiberstore Generic 1550/1310. However, now that you mention the 1G barely working, we do notice when doing an on-net or off-net speed test we get around 300M up and down, and never close to 1G. I'm not sure what a WDM mux is or how it affects things. Could that cause the bandwidth to be far less than 1G?
    – Josh Eblin
    Dec 16 '20 at 20:42
  • Check the switch port counters for FCS errors, runts and giants. Anything above .01% is very bad - if the link is slowed that much you're possibly ~5%. There could be a bad splice but it could also be dirt or scratches on the terminals. SMF is also very susceptible to overbending, causing severe attenuation.
    – Zac67
    Dec 16 '20 at 20:55
  • A WDM multiplexer can merge/split multiple distinct signals by wavelength, so you can run a number of links on a single (pair of) fiber. Very unlikely here, given the wavelengths used.
    – Zac67
    Dec 17 '20 at 5:15
  • There's no errors on the switchports. Would I see errors if there was a bad splice or something wrong with the terminals or attenuation?
    – Josh Eblin
    Dec 18 '20 at 14:10
  • If there are no errors on either side, the physical layer is working perfectly. You should add the make/model of the router/switch and possibly its config to your question - it's possible the config is preventing 10G.
    – Zac67
    Dec 18 '20 at 14:26

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