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FORE Systems ES-2810 Ethernet Switch User’s Manual
Concepts in Switching
A.4 Auto-negotiation
A.4.1 Auto-negotiation Concept
Auto-negotiation follows the IEEE 802.3u 100Base-T specification to improve the effectiveness
of dual speed links (such as the base ports and the 10/100TX Media Module). They have the
capability to work at either of their speeds (10Mbps or 100Mbps) and either of the duplex
modes (half- or full-duplex).
Normally you would need to configure the port on the switch with a specific speed or duplex
mode, but that is not required with auto-negotiation — it detects the capabilities of the other
devices (over a common link) and configures itself to use the same technology automatically;
this gives maximum efficiency.
To illustrate how auto-negotiation works, imagine two devices connected via a switch as
shown in the figure below.
Auto-negotiation allows the Switch port to select the best transmission speed and duplex
mode—based on the capabilities of the device at the other end.
The link between Device A and the Switch has Auto-negotiation enabled at both ends. Since
both ends support 100Mbps full-duplex mode, this mode is selected.
Device B is also connected to the Switch, but only supports 10Mbps half-duplex transmission.
The Switch automatically detects this and select 10Mbps half-duplex transmission for this
port.
A
Switch
B
100Base-TX
10Base-T
Auto-negotiation
100Base-TX
10Base-T
Auto-negotiation
Device:
10Base-T
Capability:
Half-duplex
Half-/full-duplex
Half-/full-duplex
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