What are the consequences of exceeding the SFP manufacturer maximum distance ? Example from Cisco, Table 2. SFP+ port cabling specifications of https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/transceiver-modules/data_sheet_c78-455693.html

Can we expect general problems, like ethernet synchronization problems, line protocol down, or others general side effects, or the only consequence will be lower bandwidth (e.g: 8/9Gbps on 10Gbps SFP, etc)?

  • There are specs in the data sheet that list out the maximum launch power and sensitivity. If you combine this information with the measured characteristics of the span (specifically attenuation) you'll get a solid idea of whether the link will function. In short, if enough light makes it past the fiber then it will most likely work. If it doesn't work, the first thing the support vendor will cite is the fact that the link is outside the official distance spec of what's supported. – rnxrx Jun 10 at 2:17

What are the consequences of exceeding the SFP manufacturer maximum distance ?

Very simple: anything between nothing over spurious data errors (effectively frame drops) to total link loss.

The link will not come up at a lower speed - with rare exceptions (2.5/5/10 Gbit/s "smartrate" ports), Ethernet doesn't try to find the best, reliable speed. It links or it doesn't. Transmitted frames are intact or they're not.

With a somewhat reasonable error rate, the link might look like it's running at a slower rate but that's only from a higher-layer point of view where packet loss and retransmissions aren't visible.

In practice, you can often exceed the specified distance, sometimes considerably (esp. with single-mode fiber). That happens when the actual optical channel is (much) better than the specs require (not unusual), or the optical modules may have better sensitivity than the minimum. Still, you need to note that you're out of spec and technically on your own, without warranty and support.

If you do run an out-of-spec link, make sure that you have more than a single pair of transceivers that work in that place - there's nothing more 'frustrating' than having a transceiver die and finding out that new ones don't work...

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