Auto-Negotiation must be enabled in 40 GbE ports when using 40GbCR4 (copper Direct Attached Cables - DACs) pluggable modules as Clause 73 of the 40 GbE standard lists it as mandatory. Though the links may come up in 40 GbE ports even without Auto-Negotiation, the best practice is to always enable Auto-Negotiation. Otherwise, there might be link instability or FCS errors.