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How come a PC can connect to a Switch using Crossover copper, and sometimes a Straight-Through Copper?

The below topology is in my Cisco Network in Packet Tracer.

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  • Newer switches can automatically detect if a cable is crossover or straight through and negotiate the correct mode in order for the host to connect correctly, also called MDIX auto. Old switches, like the 2950 does not have this feature. – user36472 Nov 15 '18 at 10:04
  • do you think the 2950 and the PC interface are the quits device? – 244boy Nov 15 '18 at 10:06
  • You always have to use Straight-Through cabling for a host to connect to a 2950 switch as it does not support MDIX auto. If you connect two 2950 switches with each other you have to use crossover. – user36472 Nov 15 '18 at 10:08
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Newer switches can automatically detect if a cable is crossover or straight through and negotiate the correct mode in order for the host to connect correctly, also called MDIX auto.

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.

Auto MDI-X is enabled by default on Cisco switches.

The Cisco 2950 does not support this feature.

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  • You should use "Auto MDI-X" - "MDI-X" is the classic switch/hub pinout without automatic crossover. ;-) – Zac67 Nov 15 '18 at 10:40
  • I actually didn't know that was possible. Do you have a config snippet? I dont have any 2950 available in our network anymore. – user36472 Nov 15 '18 at 10:42
  • No, I mean "MDIX is enabled by default..." should say "Auto MDI-X is enabled by default..." Even ancient switches use MDI-X by default (unless your throw the hardware switch or use the MDI "uplink" port). – Zac67 Nov 15 '18 at 10:44
  • Aargh, you should just have corrected that! It was copied from Cisco's own homepage. – user36472 Nov 15 '18 at 10:45

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