Auto-MDIX dilemma

Configuring Auto-MDIX on an Interface
"When automatic medium-dependent interface crossover (auto-MDIX) is enabled on an interface, the interface automatically detects the required cable connection type (straight through or crossover) and configures the connection appropriately. When connecting switches without the auto-MDIX feature, you must use straight-through cables to connect to devices such as servers, workstations, or routers and crossover cables to connect to other switches or repeaters. With auto-MDIX enabled, you can use either type of cable to connect to other devices, and the interface automatically corrects for any incorrect cabling. For more information about cabling requirements, see the hardware installation guide.

Auto-MDIX is enabled by default. When you enable auto-MDIX, you must also set the interface speed and duplex to auto so that the feature operates correctly. Auto-MDIX is supported on all 10/100 and 10/100/1000-Mbps interfaces and on 10/100/1000BASE-TX small form-factor pluggable (SFP)-module interfaces. It is not supported on 1000BASE-SX or -LX SFP module interfaces."

So you can use a straight through cable to connect two switches, but you then cannot explicitly set the speed and duplex.

"To be fair, I wasn’t aware of the auto-MDIX needing auto-negotiation issue, BUT whenever you are troubleshooting a problem you want to minimize any deviations from known working configurations to eliminate unnecessary variables. I think that I remember reading that auto-MDIX uses the same protocol as auto-negotiation and that’s why both need to be enabled. I’m not positive about that though."

Assume that “correct cabling” means a cross-over cable and “incorrect cabling” means a straight-through cable.

Local Side Auto-MDIX	Remote Side Auto-MDIX	With Correct Cabling	With Incorrect Cabling
On	On	Link up	Link up
On	Off	Link up	Link up
Off	On	Link up	Link up
Off	Off	Link up	Link down

You can refer to the table below, but it’s pretty easy to determine if a link will be up or not: at least one side needs to have auto-MDIX enabled along with auto-negotiation of speed and duplex, otherwise the link will be down.

sw1		sw2
MDIX	Speed/Duplex	MDIX	Speed/Duplex	Link Status
on	auto	on	auto	up
on	auto	on	hard-set	up
on	auto	off	auto	up
on	auto	off	hard-set	up
on	hard-set	on	auto	up
on	hard-set	on	hard-set	down
on	hard-set	off	auto	down
on	hard-set	off	hard-set	down
off	auto	on	auto	up
off	auto	on	hard-set	down
off	auto	off	auto	down
off	auto	off	hard-set	down
off	hard-set	on	auto	up
off	hard-set	on	hard-set	down
off	hard-set	off	auto	up
off	hard-set	off	hard-set	down

This makes sense because you need at least one side to be able to logically switch the pinouts of a straight-through cable to emulate a cross-over cable. Hard-setting the speed or duplex disables the auto-negotiation protocol (which auto-MDIX must utilize as well) which effectively disables auto-MDIX:

Note: The only command that I know of that will show the auto-MDIX state of an interface (other than looking at the running-configuration of the interface) is the rather verbose “show controllers ethernet-controller fax/x phy | include MDIX” command.

Note: The default setting for switch ports is to have auto-MDIX enabled. This is a pretty recent change though. IOS versions prior to 12.2(20)SE will use the default of “no mdix auto”.

“mdix auto” is the default, so it does not show in the running-configuration:
sw2(config)#do sh run int fa0/32
Building configuration…
Current configuration : 34 bytes
!
interface FastEthernet0/32
end

We can verify that auto-MDIX is on for this interface:
sw2(config)#do sh controll eth fa0/32 phy | i MD
Auto-MDIX : On [AdminState=1 Flags=0x00052248]

Let’s hard-set the speed and see what happens to auto-MDIX:
sw2(config)#int fa0/32
sw2(config-if)#speed 100
sw2(config-if)#do sh control eth fa0/32 phy | i MD
Auto-MDIX : Off [AdminState=1 Flags=0x00010A48]

Notice that our configuration does not state that auto-MDIX has been disabled:
sw2(config-if)#do sh run int fa0/32
Building configuration…

Current configuration : 45 bytes
!
interface FastEthernet0/32
speed 100
end

This verifies that hard-setting the speed and/or duplex turns off auto-MDIX for the interface.

Note: I did test to see if DTP was effected by auto-MDIX. It was not. As long as the link was up, DTP could work it’s trunking magic.

So the long and the short of it is: you can use straight-through cables to connect two Cisco switches as long as you are willing to sacrifice the ability to hard-set the speed and/or duplex on both sides of the link.

Surprisingly i tested above commands on a 2950 and a 3550-SMI switches, i could not get any info regarding auto-mdix.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_25_see/configuration/guide/swint.html