Ethernet MDI/MDI-X auto-sensing

Asset ID:	1-71-1009240.1
Update Date:	2018-05-09
Keywords:

Solution Type Technical Instruction Sure

Solution 1009240.1 : Ethernet MDI/MDI-X auto-sensing

Applies to:

Sun Fire B1600 Blade System Chassis - Version Not Applicable and later
Sun Fire V210 Server - Version Not Applicable and later
Sun Quad GigaSwift Ethernet UTP Adapter - Version Not Applicable and later
Sun Fire V240 Server - Version Not Applicable and later
Sun Fire V440 Server - Version Not Applicable and later
All Platforms
***Checked for relevance on 11-July-2014***

Goal

Many newer Ethernet switches and NICs support auto-sensing of MDI/MDI-X.
This eliminates the need for a crossover network cable when connecting one system directly to another system without a switch, or when connecting a switch to a switch not using a "up-link" port.

Solution

When using copper (UTP, RJ45) interfaces, a standard network cable is used when connecting a system to a switch.
Such a cable uses one-to-one wiring. Historically, a crossover or MDI-X cable would need to be used when directly connecting a system to another system or switch to another switch.

Acronyms:
MDI (medium dependent interface)
MDI-X (MDI crossover)
UTP (Unshielded Twisted Pair copper cable standard)
RJ45 ("Western" type ethernet connector)

Some switches have the ability to configure a port as a "up-link" or MDI-X port for connecting to another switch using a standard network cable. Some have a dedicated port for this purpose.

Interface cards in system historically needed to use a crossover or MDI-X cable to link directly to another system.

New generation Ethernet cards and switches use auto-sensing to accomplish this, making the establishment of these links more convenient. Namely, Gigabit Ethernet interfaces have this feature.
Most current Sun[TM] devices support auto-sensing of MDI/MDI-X , namely the Gigabit Ethernet interfaces for systems and switches.

IMPORTANT: MDI/MDI-X cable detection will not work unless auto-negotiation is on or enabled.

If auto-negotiation is disabled, auto-MDI/MDI-X pin signal configuration will also be disabled. This means that you will have to use a cross-over cable to connect two systems back to back. If you are connecting to a switch, and the switch doesn't have the ability to configure an up-link port or have a hard wired up-link port, you will also need to use a MDI-X cable.

An alternative is to leave auto-negotiation enabled (this is the default setting) but reduce the subset of permitted modes to the single mode that you want to use.

For example, to force an uplink to 1000BASE-T and retain MDI/MDI-X auto- sensing, have the port set to negotiate but with "1000full" as the only capability.

See also:
Technical Instruction <Document: 1005843.1> - Sun[TM] GigaSwift Gigabit Ethernet Adapters First Checks
Technical instruction <Document: 1001843.1> - bge Gigabit Ethernet First Checks

MDI, MDI-X, crossover, uplink, up-link, up, link, auto, negotiation, back, gigabit, ethernet, utp, rj45, down, cable
Previously Published As 77124Change History
Date: 2004-07-05
User Name: 25440
Action: Approved
Comment: Minor edits ("Ethernet" throughout instead of randomly "Ethernet" and "ethernet", "auto-negotiation" rather than "auto-negotiation", "autonegotiation" and "autoneg", and so on). Publishing.
Version: 0
Date: 2004-07-05
User Name: 25440
Action: Accepted
Comment:
Version: 0

References

<NOTE:1397422.1> - ce Gigaswift support document
<NOTE:1395856.1> - Product Support Document for bge Gigabit Ethernet
<NOTE:1400599.1> - Ethernet Product Page
<NOTE:1005843.1> - Sun[TM] GigaSwift Gigabit Ethernet Adapters First Checks
<NOTE:1001843.1> - bge Gigabit Ethernet - First Checks

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