Duplex negotiation over 100Base fibre

Mon Feb 29 18:01:12 UTC 2016

> On Feb 29, 2016, at 11:07 AM, Jason Lixfeld <jason+nanog at lixfeld.ca> wrote:
> 
> Hello,
> 
> My understanding is that for 1G and 10G optical networks, there is no concept of half-duplex mode, but I’m unclear about half duplex in the 100M optical world.  Specifically, if I connect two 100Base-LX (or BX) transceivers together, is there a requirement for the controller(s on either side) to select between full or half duplex, either by static configuration, or auto-negotiation?

I can say even at 1G speeds, the fiber is just the medium, the underlying data stream can still be half-duplex or require autonegotation or the lack of it. I recall many years ago having to adjust settings for GSR <-> Foundry negotiation and how it would vary depending on the device, so if you changed from GSR -> 6500/7600, you may have to make changes to maintain link.

Having a standard device/switch/config will help you here.

Most media converters I’ve seen have a switch on the side that lets you set if it should advertise autonegotation, take the super-cheap MC220L as an example, it has a switch for this purpose.

After many years of hard coding things to gig/full, i’ve started to see the autonegotation work much better and require less custom coding of the config.  This doesn’t stop someone from plugging in really-old stuff, and we still see people connecting at 10/half or 10/full when I really wonder what they are connecting.

- Jared