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RX/TX on Copper and Fiber cable

Posted on 2016-08-14
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Last Modified: 2016-09-14
On Fiber cable , even though the end of the cable is connected to one port on the switch (fiber cable port), but it Transmits and Receives Frames at the same time
On copper cable if I am not wrong there are about 5 or 6 wires , twisted in pairs . Which of them do Transmit and Receive Frames?
and if there are TX/RX capabilities on copper cable then do we need to configure UDLD/Loop Guard ?

Thank you
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Question by:jskfan
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12 Comments
 
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by:John Hurst
John Hurst earned 125 total points (awarded by participants)
ID: 41755409
Here is a standard layout for Ethernet cable that shows the transmit and receive wires. Not all wires are used for Ethernet. Twisting the pairs keeps inter-conductor capacitance low.

http://www.incentre.net/tech-support/other-support/ethernet-cable-color-coding-diagram/
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Michael Ortega (Internetwerx, Inc.) earned 125 total points (awarded by participants)
ID: 41756208
Loop Guard is there to protect from switching loops. UDLD deals with unidirectional links. Neither really have anything to do with the first part of your question.

MO
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by:jskfan
ID: 41764245
UDLD will alert the Switch when one a link (TX or RX) is broken and the Switch puts the port in Err-Disabled

Loopguard when enabled then if a switch Forwards BPDUs but does not receive any, it will turn into loop-inconsistent state . Though I wonder why it cannot turn into Blocking State

--Now if UDLD and Loopguard are applied both at the same time will they accomplish the same thing? if so then why not just apply one of them ?
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by:noci
noci earned 125 total points (awarded by participants)
ID: 41764305
Loop guard needs to be enabled on BOTH ends of a cable and only makes sense where loops can exist. (like on links between multiple switches). So just enabling them on one end is what is says an inconsistent configuration of the network.
Blocking will only start on an interface when the switches find out that there is a loop between them.
(A loop will cause multiplication of traffic through the loop, and this self amplifying so the network will crash soon after a packet starts looping, aka broadcast storms because broadcast packets often initiate such an event).

UDLD and Loopguard ARE DIFFERENT settings for different problems and provide different solutions.
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by:jskfan
ID: 41764375
Well in another thread I just closed, the Expert stated that UDLD is used only on Fiber and Loop Guard on Copper..
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LVL 16
ID: 41764592
Can you provide us with the link to that question?

MO
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by:jskfan
ID: 41764724
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by:noci
noci earned 125 total points (awarded by participants)
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Ah ok but it is not EITHER / OR... he meant UDLD works on Fiber links because that is easier to detect through fiber.
STP works on BOTH.  So i would say that is a misinterpretation of the answer given there.

RSTP (Rapid STP is converging with line speed, where Classic STP only checks every 30 seconds and is slow).
Most important RSTP and STP CAN be mixed to support old devices.

Now regarding VLAN's/LAN's...
L2 Switching, needs a mechanism to prevent loops from occuring either through misconnected cables or through use of redundant (standby) links. A strict star shaped topology with strict wiring & interface policies (disabled if unused) does not need STP. Otherwise it is a rather cheap insurance policy against network meltdowns that are hard to trace. RSTP is prefered there.  A VLAN setup is just a setup where multiple LAN's may cross a given wire/switch, so ANY VLAN can take out a link (flooding) or switch (cpu power) if misconfigured. In that sense a VLAN setup is more prone to network meltdowns then regular ones.  

L3 Switching is actually a Router bolted onto a L2 switch, where the L2 switch engine consults the L3 Routing engine for forwarding rules, in stead of just using the MAC addresses. So yes it may fail differently and less on broadcast storms.
(L3 networks also tend to be more star shaped, which means less/no chance of loops).
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by:Craig Beck
Craig Beck earned 125 total points (awarded by participants)
ID: 41769619
UDLD isn't required on copper links but is on fibre links.  In a fibre cable traffic only goes one way.  Consider a fibre cable directly between two switches.  The leg labelled 'A' will transmit from switch 1 to switch 2, while the fibre labelled 'B' will transmit from switch 2 to switch 1.  Flipping that over, Switch 1 will receive traffic from Switch 2 on fibre leg 'B'...

With copper you have 2 wires to complete the transmit circuit and two wires to complete the receive circuit.  Both of these circuits need to be closed for each end to acknowledge a link because it's electrically-driven instead of light-driven, so unidirectional links are detected by nature of the medium in copper anyway.

Loopguard can be configured on any media type; fibre or copper.  It's not the same as UDLD nor is it the copper equivalent.  Loopguard is used by STP to help mitigate against L2 loops.  UDLD is used to drop unidirectional links so traffic isn't blackholed.  It's extremely important especially where routing is concerned, for example.
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by:noci
ID: 41770663
That said, and i do agree, i have seen unidirectional Copper also (defective receiver circuitry on one side, the copper connection was)  intact. So blackholing does occur, but broken fibers are more common.
These connections failures were more common when people connected ISDN to Ethernet switches... i agree to that being foolish but it was done.

Also there is a single fiber with bi-directional data transmission, it is used in residential area  fiber deployments  to connect homes to the internet. I agree not often seen outside of that realm, but single fiber connections with bi directional data transfer does exist.
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by:Craig Beck
Craig Beck earned 125 total points (awarded by participants)
ID: 41770822
Unidirectional links are extremely uncommon when using BX optics (bidirectional over a single fibre-strand).  It's used a lot in the UK to provide WAN circuits, LAN extensions, etc, for businesses and large enterprises.
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by:noci
ID: 41797498
All contributed.
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