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UPDATE: I just clarified with my friend, he actually meant, the traffic will be passed through from the LAN to WAN port, it's specific to device, in our case it's the Symantec Web Gateway Appliance. Now it's that true?

Original: If a cross cable is used to connect from an device to another, say firewall to a web gateway. When the web gateway is down (switched off, but cable still connected), the traffic sent to the web gateway will be "routed" back to firewall (maybe like a loopback?). But it won't for a straight cable.

I've heard about this from a friend, but I don't quite believe it, so I wanted to see if anyone else have heard about it.

EDIT: Yes, no traffic is transported by the controller or onboard MAC, but could the PHY be "shorted" electronically, TX shorted to RX, so data just looped over? Maybe it's an undocumented "feature" of some devices?

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closed as not a real question by larsks, Sven, John Gardeniers, Ward, Bryan Feb 10 '12 at 8:27

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You need a new friend. –  John Gardeniers Feb 9 '12 at 20:49
    
Guys, I managed to test it yesterday, it did pass the packet through from LAN to WAN. I think it's a special NIC with 2 port, which will keep the packet through if it's down. I see it with my own eyes. It's also mentioned in the manual –  faulty Feb 11 '12 at 12:38

3 Answers 3

This is nonsense. When a device is down, no traffic is transported, period.

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There are 2 types of cables:

Crossover is to connect two devices without a hub or a switch. Loopback is used to test the local port of a device.

Some equipments offer also remote loopback that is used to test the entire line from local equipment, until the remote end port. The configuration of such devices is done out of band or by someone that has access at the remote end.

If the remote equipment is down, the link can be in 2 states:

  • down, this means that the remote equipment is not sending the carrier signal and the local endport will consider the link to be down and will remove the route from the route cache
  • up, this is happening when the remote end has wake on LAN enabled. The remote end will send the carrier signal, but it will not send any data. Remote end will just wait to receive a WoL packet.

There is no difference between the straight cable or the cross over besides that it will make a link to work or not.

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Your friend is wrong. To clarify the terminology:

A standard ethernet cable is used to connect a device (router, printer, laptop, pc etc) to a switch.

A crossover cable is used to connect two devices (eg, a pc direct to a router)

Here is wikipedia's page on Ethernet Crossover Cables. As this page points out, most modern NICs and switches now implement Auto-MDIX, crossover cables aren't really needed any more.

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Thanks. I know what is a cross cable. –  faulty Feb 10 '12 at 2:41

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