Auto-negotiation and Fiber SFP Interfaces

[sherrick]

We were replacing a router at a location where our WAN provider has a 1000BaseLX handoff with auto-negotiation OFF. Our previous router was an ISR4321 that had auto-negotiation ON, so we duplicated that configuration on our new router - an ASR920. However when swapping the connection over using the same SFP and the same fiber jumper, the ASR920 would not come online until we turned auto-negotiation OFF. 

I had a hard time finding documentation on auto-negotiation on fiber SFP interfaces. I have two questions:

1. What exactly does auto-negotiation do in the context of fiber SFPs (obviously it's nothing to do with duplex as with copper).
2. Why does it seem these two different Cisco platforms (ISR4321/ASR920) handle auto-negotiation differently so as to cause the issue above?

Thank you

[balaji.bandi]

Most of the Optic driven by ASICs that are fixed speed - Some Ports like SFP+ can offer you can use 1GB Optic or 10GB optic here.

If you are handing over using MPLS or SP provider - if the device not connected directly with dark fibre - suggested config always nonegotiate will give you decent outcome (most cases i have seen in real world).