Can a 10G SFP+ optic negotiate down to link with a 1G SFP at the far end?

Question

The poster asked whether a 10G SFP+ SR or LR optic in one switch can auto-negotiate down to 1G to link over fiber with a 1G SFP in another switch, and if not, how to attach a handful of 1G optical connections from a 1G switch to a 10G switch. They understood that Cisco SFP+ optics run only at 10G and Cisco SFP optics only at 1G, and believed SFP+ modules in general cannot drop to 1G because the SFP+ specification covers roughly 4.25 to 16 Gbps; they wanted the general principle confirmed rather than an answer for a specific model.

Hardware: Cisco UCS server with Intel X520 adapter, generic 10G switch (unspecified)
Transceivers: SFP-10G-SR, GLC-SX-MMD, Intel E10GSFPSR, Intel E10GSFPLR
Form factor: SFP, SFP+

Symptoms / messages:

10G SFP+ cannot link to a 1G SFP at the far end
speed mismatch between 1G and 10G optics

wavesmith · Accepted Answer

Optical transceivers never auto-negotiate, regardless of vendor: they are driven by fixed-speed ASICs. A few dual-rate 100M/1G optics exist using two VCSELs, but no 1G/10G equivalents; instead, many 10G SFP+ ports accept 1G SFPs, so check the platform's compatibility matrix.

qsfpwolf · Answer

A 1G SFP will physically fit an SFP+ port, but wheether it works depends on the transceiver's firmware combined with the IOS on the specific hardware; consult the compatibility matrix before buying to avoid an expensive miistake.

waverunner63 · Answer

Intel E10GSFPSR and E10GSFPLR are dual-rate 1G/10G optics - I ran them in an Intel X520 in a UCS server against Cisco GLC-SX-MMD and later SFP-10G-SR, with no configuration changes.

coreguru · Answer

A later reader planned to use an X520 with those dual-rate optics in a UCS C220 M4 to reach a 1 Gbps single-mode network hosting a virtual CSR router.

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