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

Martin T m4rtntns at gmail.com
Mon May 6 06:07:49 EDT 2013


Juniper routers support enabling(this is the default setting) and
disabling auto-negotiation both on 1000BASE-T(copper) and
1000BASE-X(optical) interfaces. Auto-negotiation on copper ports makes
sense because copper ports(for example on tri-rate DPC's on MX960)
support 10BASE-T and 100BASE-TX modes besides 1000BASE-T, 1000BASE-T
supports both full- and half-duplex modes according to IEEE 802.3z and
master/slave relationship between two ports needs to be determined for
negotiating the clock settings. However, what is negotiated between
two directly connected 1000BASE-X ports when auto-negotiation is
enabled on both ports? I mean optical transceivers rated to 1Gbps do
not support backward compatibility to lower speeds and are there
optical transceivers out there that support half-duplex mode(it's
supported according to IEEE 802.3 In a nutshell, why is
auto-negotiation needed on 1000BASE-X ports?


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