[j-nsp] auto-negotiation on 1000BASE-X ports

[Olivier Benghozi]

1000Base-X can negotiate flow control.

But, an interesting part of autoneg is Remote Fault Notification: one of the fibers in your 2 fibers link breaks, and the link becomes unidirectional; the side that sees its receiving fiber down sends a frame to notify the other side (which didn't see anything special) that the link is down (so this side will also show the link as "down", whereas it receives proper signal).
Without this, when a single fiber breaks, to detect (slower) the problem and prevent unidirectional GE links, you have to rely on protocols running at a higher level: specialized ones (Cisco's UDLD, OAM), on routing protocols, or on LACP (which can be used on a single link for this purpose, as would describe http://kb.juniper.net/InfoCenter/index?page=content&id=KB13314).

This also exists in 10GE links as Link Fault Signaling.


regards,
Olivier


> supported according to IEEE 802.3 22.2.4.4.2)? In a nutshell, why is
> auto-negotiation needed on 1000BASE-X ports?