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36 Dell PowerConnect 28xx Systems User Guide
Port Default Settings
The general information for configuring the device ports includes the short description of the auto-
negotiation mechanism and the default settings for switching ports.
Auto-Negotiation
Auto-negotiation enables automatic detection of speed, duplex mode and flow control on switching
10/100/1000BaseT ports. Auto-negotiation is enabled per port by default.
Auto-negotiation is a mechanism established between two link partners to enable a port to advertise its
transmission rate, duplex mode and flow control (the flow control by default is enabled) abilities to its
partner. The ports then both operate at the highest common denominator between them.
If connecting a NIC that does not support auto-negotiation or is not set to auto-negotiation, both the
device switching port and the NIC must be manually set to the same speed and duplex mode.
If the station on the other side of the link attempts to auto-negotiate with a device 10/100/1000BaseT
port that is configured to full duplex, the auto-negotiation results in the station attempting to operate in
half duplex.
MDI/MDIX
The device supports auto-detection of straight through and crossed cables on all switching
10/100/1000BaseT ports. The feature is part of the Auto-negotiation and is enabled when Auto-negotiation
is enabled.
When the MDI/MDIX (Media Dependent Interface with Crossover) is enabled, the automatic
correction of errors in cable selection is possible, making the distinction between a straight through cable
and a crossover cable irrelevant. (The standard wiring for end stations is known as MDI (Media
Dependent Interface), and the standard wiring for hubs and switches is known as MDIX.)
Flow Control
The device supports 802.3x Flow Control for ports configured with the Full Duplex mode. By default,
this feature is enabled. It can be enabled per port. The flow control mechanism allows the receiving side
to signal to the transmitting side that transmission must temporarily be halted to prevent buffer
overflow.
Back Pressure
The device supports back pressure for ports configured to half duplex mode. By default, this feature is
disabled. It can be enabled per port. The back pressure mechanism prevents the transmitting side from
transmitting additional traffic temporarily. The receiving side may occupy a link so it becomes
unavailable for additional traffic.