Features of the Nortel Ethernet Routing Switch 4500 Series 137corrects the polarity of the received data, if the port detects that the polarityof the data is reversed due to a wiring error. If autonegotiation is disabled,auto-polarity is not active.Autosensing and autonegotiationThe Nortel Ethernet Routing Switch 4500 Series are autosensing andautonegotiating devices:• The term autosense refers to the ability of a port to sense the speed ofan attached device.• The term autonegotiation refers to a standard protocol (IEEE 802.3uor 802.3z or 802.3ab) that exists between two IEEE-capable devices.Autonegotiation enables the switch to select the best speed and duplexmodes.Autosensing occurs when the attached device cannot autonegotiate oruses a form of autonegotiation that is not compatible with the IEEE 802.3zautonegotiation standard. If it is not possible to sense the duplex mode ofthe attached device, the Nortel Ethernet Routing Switch 4500 Series revertsto half-duplex mode.When autonegotiation-capable devices are attached to the Nortel EthernetRouting Switch 4500 Series, the ports negotiate down from 1000 Mb/sand full-duplex mode until the attached device acknowledges a supportedspeed and duplex mode.Custom Autonegotiation AdvertisementsIn the Nortel Ethernet Routing Switch 4500 Series, you can use the CustomAutonegotiation Advertisements (CANA) feature to control the speed andduplex settings that each Ethernet port of the device advertises as part ofthe autonegotiation process.Without CANA, a port with autonegotiation enabled advertises all speedand duplex modes supported by the switch and attempts to establish a linkat the highest common speed and duplex setting. By using CANA, youcan configure the port to advertise only certain speed and duplex settings,thereby establishing links only at these settings, regardless of the highestcommonly supported operating mode.CANA provides control over the IEEE802.3x flow control settings advertisedby the port, as part of the autonegotiation process. You can set flow controladvertisements to Symmetric, Asymmetric, or Disabled.You may not want a port to advertise all supported speed and duplex modesin the following situations:• If a network can support only a 10 Mb/s connection, you can configurea port to advertise only 10 Mb/s capabilities. Devices that usesNortel Ethernet Routing Switch 4500 SeriesOverview — System ConfigurationNN47205-500 02.01 Standard5.0 23 February 2007Copyright © 2007, Nortel Networks Nortel Networks Confidential.
