[AusNOG] Megaport - Link Negotiation

[Glen Turner]

Damian Guppy lit pixels to say:
> 
> I would assume it's actually a function of the SFP module and not of the
> router/switch its plugged into, since as has been said, you can plug a 1G
> SFP module into a SFP+ slot and it works at 1G. The SFP+ module would just
> report its operating as a 1G module.

I wish it were that simple.

A SFP+ slot is electrically compatible with SFP. However driving a SFP in 
a SFP+ slot requires programming different to driving a SFP in a SFP slot, 
particularly if the programmer used the hardware rate select line rather 
than a SFP rate select register. There's a fine Finisar datasheet on how 
to do this for their SFP+, and every manufacturer has adopted that 
technique. Needless to say some manufacturers still can't manage to get 
it right, and of interest to AusNOG the MikroTik equipment does not 
correctly set up a multi-rate SFP[1] in a SFP+ slot (a software issue, so 
hopefully they'll correct it).

You cannot drive a 10GE signal into a GbE receiver. If you disable 
autonegotiation you may get link, but that doesn't imply the ability to 
receive frames.

A very few SFP+ can output both 10Gbps and 1Gbps signalling, speed 
selectable through the SFP+ registers. Perhaps in the long run the cost of 
these will fall enough for them to be widely available, but at the moment 
they are a curiousity. Again, these require the SFP+-using software to 
correctly set up the SFP+'s registers, and that shouldn't be 
assumed.

Support for exotic SFPs is becoming more and more of an issue as SFPs 
gain more and more features (eg, DWDM frequency selection, ethernet OAM).

-glen

[1] ie: most SFPs, as they'll support 1Gbps for GbE, 2Gbps for FC, etc 
even if labelled by the distributor as GbE.

-- 
Glen Turner <http://www.gdt.id.au/~gdt/>