A: Ethernet Cabling Pinouts
Which of these uses a straight-through cable: a cable between two switches? Two routers? Two PCs? A switch and a router? A switch and a hub? The list goes on for a bit. This latest question about Ethernet cabling pinouts makes you remember the combinations and apply them. Check the question, and come back here to check your answers. Post with any related clarifications or comments.
Letter Answers
C, E
Explanation
Ethernet uses crossover cables between two devices that transmit on the same pin numbers. The crossover cable crosses the wires inside the cable, so that the wires used by one device to transmit data connect to the neighboring device’s pins used to receive data. In 10 and 100 Mbps Ethernet, that means that the pair connected to pins 1 and 2 connect to pins 3 and 6 on the other end of the cable, and vice-versa.
To find cases where the devices on both ends of the link uses the same pins to transmit, you need to remember the pins used by each device. For instance, Ethernet NICs in end user devices transmit on pins 1,2, and switches transmit on pins 3,6 normally. The following list notes the two groups: devices that transmit on 1,2 and the devices that transmit on 3,6:
- 1,2: PCs, servers, most end-user devices, routers, wireless APs
- 3,6: switches, hubs
As a result, for this question, the links that should use crossover cables are links 5 and 9, making answers C and E correct.
Hi Wendell
Your blog Is so much helpful for a beginners like me. Thank you!
I have one question:
Do they as questions like this in the real CCENT or CCNA exam where only one of the two options is right and that is why we have to select two answers?
Roopali
Hey Roopali,
Glad you’re finding the blog material helpful!
The real exam will have questions for which you must select multiple answers, but Cisco tells you the number of right answers. I’m not sure if that’s exactly what you were asking, though. Cisco does get creative in how they ask questions, but not for the purpose of making the question confusing, but just to make sure that you know the specific info or can apply a specific bit of knowledge.
Hope this helps!
Wendell
Hi Wendell
I’m confuse why 6 & 10 part of c & e ? The crossovers are only 5 & 9?
You´re right Anna, only links 5 and 9 need a crossover cable. If you re-read the question says: Wich answers list AT LEAST ONE link…..
Hello Mr.Wendell,
In “CCENT/CCNA ICND1, Third Edition” in page 60 and 61 at “Transmitting Data Using Twisted Pairs” you said that why wires are twisted together.
But I don’t understand what is your mean about “opposite direction”?
Because pair 2(in T568-B) and pair 3(in T568-A) is used for transmit in one direction while are twisted together.
Hi Reza,
Sorry it took me so long.
So, to transmit, you need an electrical circuit. To create the circuit, the devices use a pair of wires. Each device connects to both wires, so that the electrical current flows down one wire, through a device, back over the other wire, etc. Without a circuit, the electricity can’t flow, so the devices create a circuit (loop) with the two wires.
Then, on one circuit, on one pair, the first device acts as the sender by varying the electrical current. The second device on the other end acts as the receiver, watching for the changes to the electrical signal. For the other pair, the second device acts as sender. and the first device acts as the receiver.
Hope this helps.
Wendell
Answer “C” is correct. Connections between devices that transmit and receive on the same pins require crossover cable.
For answer “E” to be correct, BOTH cable #9 AND cable #10 must also be a crossover cables. As configured, cable #10 must be straight-thru cable — it cannot be a crossover cable.
For “E” to be a valid answer, cables 9 *and* 10 must be crossover cables.
Cable 9 connects devices that transmit and receive on similar wire pairs. However, hubs and PC do not transmit and receive on similar wire pairs. Therefore cable cannot be a crossover cable.
Hi Aristopele,
Thanks for the comment.
On your comment regarding answer E, I disagree with you. For E to be a correct answer, only one of the two cables needs to be a crossover cable. That what the phrase “at least one of the…” means in the question stem. That is, the answer is correct if one or more of the cables meets the criteria.
Wendell
oh, I didn’t see this comment “at least one of the …” which is part of the question..thanks..
no problem.
Hello Mr.Wendell,
Is it Link 6 and 9 instead of 5 and 9 as crosover cable?
Link 6 between AP and SW2 ?
Rott
Hi Rott,
Nope. The link from the switch to the AP will be a straight-through cable.
Wendell
Hi, Wendell Sir,
In your answer you have listed devices who transmit on pin 1 2 and receive on 3 6, PC transmit on 1 2 and Hub Transmit on 3 6, So, why do we need cross over cable on PC to Hub?! It would be straight-through.
Asu,
I agree, a PC connected to a hub would use a straight-through cable. However, I don’t see where this post claims otherwise – can you give me a few literal words from where you see that, so I can search the page and find it? Or maybe a few more words as to where in this post you think it says otherwise? But I agree that hub-PC is straight-through.
Wendell