Switch#interface GigabitEthernet0/xx
Switch(config-if)#mdix auto ←デフォ
12.2(25)FX以降で対応済みで、このIOSのリリースがSeptember 2005ってなってるから、保守期限考えたら今（2011初頭）動いているスイッチのほとんどはもう対応している、かな？
注意点としては「Usage Guidelines」に書いてあるけど
When you enable auto-MDIX on an interface, you must also set the interface speed and duplex to auto so that the feature operates correctly.
speedやduplexでネゴを固定にしているときはauto-MDIXは機能しないから、固定にしなくちゃいけない場合にはストレートやクロスケーブルを使い分けましょうｗ
http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_37_se/command/reference/cli1.html#wp2786606
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【mdix auto】
Use the mdix auto interface configuration command to enable the automatic medium-dependent interface crossover (auto-MDIX) feature on the interface. When auto-MDIX is enabled, the interface automatically detects the required cable connection type (straight-through or crossover) and configures the connection appropriately. Use the no form of this command to disable auto-MDIX.
mdix auto
no mdix auto
[Syntax Description]
This command has no arguments or keywords.
[Defaults]
Auto-MDIX is enabled.
[Command Modes]
Interface configuration
[Command History]
Release Modification
12.2(25)FX
This command was introduced.
[Usage Guidelines]
When you enable auto-MDIX on an interface, you must also set the interface speed and duplex to auto so that the feature operates correctly.
When auto-MDIX (and autonegotiation of speed and duplex) is enabled on one or both of connected interfaces, link up occurs, even if the cable type (straight-through or crossover) is incorrect.
Auto-MDIX is supported on all 10/100 and 10/100/1000 Mb/s interfaces. It is not supported on 1000BASE-SX or -LX small form-factor pluggable (SFP) module interfaces.
[Examples]
This example shows how to enable auto-MDIX on a port:
Switch# configure terminal
Switch(config)# interface gigabitethernet0/1
Switch(config-if)# speed auto
Switch(config-if)# duplex auto
Switch(config-if)# mdix auto
Switch(config-if)# end
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Switch(config-if)#mdix auto ←デフォ
12.2(25)FX以降で対応済みで、このIOSのリリースがSeptember 2005ってなってるから、保守期限考えたら今（2011初頭）動いているスイッチのほとんどはもう対応している、かな？
注意点としては「Usage Guidelines」に書いてあるけど
When you enable auto-MDIX on an interface, you must also set the interface speed and duplex to auto so that the feature operates correctly.
speedやduplexでネゴを固定にしているときはauto-MDIXは機能しないから、固定にしなくちゃいけない場合にはストレートやクロスケーブルを使い分けましょうｗ
http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_37_se/command/reference/cli1.html#wp2786606
---
【mdix auto】
Use the mdix auto interface configuration command to enable the automatic medium-dependent interface crossover (auto-MDIX) feature on the interface. When auto-MDIX is enabled, the interface automatically detects the required cable connection type (straight-through or crossover) and configures the connection appropriately. Use the no form of this command to disable auto-MDIX.
mdix auto
no mdix auto
[Syntax Description]
This command has no arguments or keywords.
[Defaults]
Auto-MDIX is enabled.
[Command Modes]
Interface configuration
[Command History]
Release Modification
12.2(25)FX
This command was introduced.
[Usage Guidelines]
When you enable auto-MDIX on an interface, you must also set the interface speed and duplex to auto so that the feature operates correctly.
When auto-MDIX (and autonegotiation of speed and duplex) is enabled on one or both of connected interfaces, link up occurs, even if the cable type (straight-through or crossover) is incorrect.
Auto-MDIX is supported on all 10/100 and 10/100/1000 Mb/s interfaces. It is not supported on 1000BASE-SX or -LX small form-factor pluggable (SFP) module interfaces.
[Examples]
This example shows how to enable auto-MDIX on a port:
Switch# configure terminal
Switch(config)# interface gigabitethernet0/1
Switch(config-if)# speed auto
Switch(config-if)# duplex auto
Switch(config-if)# mdix auto
Switch(config-if)# end
---