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Tuesday, February 1, 2011

Configuring Port Negotiation

Gigabit Ethernet interfaces use port negotiation to negotiate various parameters that relate to the physical operation of the interface. Port negotiation is used to exchange the following information:

  • Duplex settings All Cisco gigabit interfaces operate only in full-duplex mode.

  • Flow control capabilities Allows an interface to advertised whether or not it can support flow control features.

  • Remote fault information

Notice in the preceding list that speed is not included in the list of auto-negotiated features, because gigabit Ethernet interfaces operate only at gigabit speeds. There is no requirement to negotiate speed.

NOTE

Cisco Catalyst switches also support 10/100/1000BASE-T auto-sensing interface, which obviously can operate at different speeds. On these ports, the switch first detects the clock rate on the interface. If a speed of 10/100 Mbps is detected, the auto-negotiation procedures for Fast Ethernet are invoked. If a speed of 1000 Mbps is detected, the port negotiation procedures for gigabit Ethernet are invoked.

 

On Cisco Catalyst switches, when configuring gigabit Ethernet interfaces, the following rules apply:

  • You cannot modify speed or duplex settings on GBIC-based interfaces.

  • You can modify speed and duplex settings for 10/100/1000BASE-T interfaces.

  • You can disable port negotiation for 1000BASE-SX, 1000BASE-LX and 1000BASE-ZX GBIC-based interfaces.

  • You cannot disable port negotiation for 1000BASE-T GBIC-based interfaces.

  • If you enable port negotiation on a gigabit Ethernet interface, you must ensure port negotiation is configured on the other side of the connection. If port negotiation is enabled on one side but disabled on the other side, the interface configured for port negotiation does not come up.

  • If port negotiation is disabled on both sides of a gigabit Ethernet connection, the link comes up; however, some features such as flow control might not be supported depending on your configuration.

By default, gigabit Ethernet interfaces have port negotiation enabled, and for most situations, this configuration should not require modification. The only time you need to disable port negotiation is if you are connecting to a device that does not support port negotiation.

To configure port negotiation on Cisco IOS, the speed interface configuration command is used, even though port negotiation on gigabit Ethernet has nothing to do with speed. On GBIC-based interfaces, the speed command has the following syntax:

Switch(config-if)# [<span class="docEmphStrong">no</span>] <span class="docEmphStrong">speed nonegotiate</span>

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