uplink ports and actual sfp port in switches ( please help )

[SJ K]

Dear all,

I am reading around to see the differences between uplink ports and normal ethernet ports. It seems to be uplink port just reverse the rx,tx pins and remove the needs of using a crossover cable.

I have done a simple representation of using the below (normal ethernet ports)

PC		SWITCH		ROUTER
RX	straight	TX	straight	RX
RX	straight	TX	straight	RX
TX	straight	RX	straight	TX
TX	straight	RX	straight	TX

I assumed that the SWITCH ethernet port pins are naturally arranged as opposite from the PC so that a straight cable can be used and the same applies to a router port.

Q1) Hence from PC to Router ethernet port, I will need a cross cable, am I right ?

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Then I chance upon this website http://duxcw.com/faq/network/uplink.htm which states "A PC can be connected to an uplink port with a crossover cable"

As per my assumption above, a router has normal ethernet ports and uplink ports (for connecting to the wan/modem), since the normal ethernet ports of a router are in the order of (RX,RX,TX,TX) as shown above, I assumed the uplink port is (TX,TX,RX,RX).

PC		ROUTER normal ethernet port				PC		ROUTER uplink port
RX	cross	RX				RX	straight	TX
RX	cross	RX				RX	straight	TX
TX	cross	TX				TX	straight	RX
TX	cross	TX				TX	straight	RX

Q2) So how can the PC connect to the Router's Uplink port using a crossover cable ? Shouldn't it be using straight cable instead if it is trying to connect to a router's uplink port ?

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Q3) Where connecting switches (switch 1 to swtich2) using uplink port, can I say that a straight cable will be connected to switch 1 uplink port on 1 end, and a normal ethernet port in switch 2 on another end ?

SWITCH#1 ethernet port		SWITCH#1 Uplink port		SWITCH#2 ethernet port
TX		RX	straight	TX
TX		RX	straight	TX
RX		TX	straight	RX
RX		TX	straight	RX

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Q4) What is the purpose of an SFP port in a switch ? is it use as an uplink port ? Where does it actually uplink/connect to ?

Q5) If i connect a switch Uplink port to another switch Uplink port, does it means that I have to use a crossover cable ? does SFP fiber come with "crossover" type too ?
 

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Q6) If an uplink port in a switch is use to connect to a router (can we?), do i need to use a crossover cable then ? since a normal switch ethernet port will use a straight cable to connect to a router port ? as shown below

SWITCH		ROUTER		SWITCH UPLINK		ROUTER
TX	straight	RX		RX	cross	RX
TX	straight	RX		RX	cross	RX
RX	straight	TX		TX	cross	TX
RX	straight	TX		TX	cross	TX

============================================================

Q7) Last but not least, if a design connect two switches using SFP uplink ports on both side, what would be the main purpose ? to increase the amount of ports ?  Why not stack them then ?

Sorry for the lengthy post, will really appreciate if some gurus can shed some light on the above.

Thanks a million.

Regards,
Noob

[acampbell]

Hi Noob,

The best way to learn about inerconnecting devices is to
understand their functions:-

In datacomms there are only 2 types of funtion:-

1) Data Terminating Equipment - Known as DTE
2) Data Circuit Terminating Equipment - Known as DCE

DTE - PCs, Printers, Laptops, Cameras, and IMPORTANTLY ROUTERS

DCE - Switches, Hubs

DTE to DCE STRAIGHT
DTE to DTE CROSSOVER
DCE to DCE CROSSOVER

Switch to switch (or Hub) is known as an UPLINK so it should be a Crossover (DCE to DCE)
Switch to Router should just be a straight (DCE to DTE)

Nowadays a lot of kit can auto detect whether it needs a straight or crossover
whhen you plug in the cables. For me I am old :) so I like to use the correct type of cable
for the function I am am interconnecting.

I hope this helps.
Regards
Alex

[SJ K]

Hey Alexandra,

Thanks for replying. I am glad you saw this post. Actually i am reading these posts which leds me to think that an uplink port actually reverse the pinsout so that we can use a straight cable to connect.

Please see link below and let me know if I am right.
http://duxcw.com/faq/network/uplink.htm
http://compnetworking.about.com/od/homenetworking/f/uplink-port.htm
http://www.speedguide.net/faq/what-is-the-uplink-port-on-a-router-hub-or-switch-85

Regards,
Noob

[acampbell · Accepted solution]

Hi Noob,

On some kit like netgear hubs etc, you may find a little button or
slider switch which may be labelled MDI or MDI(X) this allows a
straight cable to be used between 2 Hubs or switches (Both DCEs).

Some Cisco switches/routers will allow you to do the same but in software

!
interface fastethernet 1/0/1
mdix auto
!

====================================

DTE devices like say your laptop PC NIC card has an MDI (Medium Dependant Interface).

Physically this wires out as ( For 10/100M):-

RJ45 PIN
1  -- TRANSMIT +ve
2  -- TRANSMIT -ve
3  -- RECEIVE +ve
6  -- RECEIVE -ve

DCE devices like hubs & swiches have MDI-X (Medium Dependant Interface - Crossedover).

Physically this wires out as ( For 10/100M):-

RJ45 PIN
1  -- RECEIVE +ve
2  -- RECEIVE -ve
3  -- TRANSMIT +ve
6  -- TRANSMIT -ve

So lets connect the device together with a STRAIGHT cable ( Function DTE to DCE)

PC/NIC = DTE = MDI                                SWITCH = DCE = MDI(X)
RJ45 PIN                                           RJ45 PIN
1  -- TRANSMIT +ve -------------------------------- 1  -- RECEIVE +ve
2  -- TRANSMIT -ve -------------------------------- 2  -- RECEIVE -ve
3  -- RECEIVE +ve --------------------------------- 3  -- TRANSMIT +ve
6  -- RECEIVE -ve --------------------------------- 6  -- TRANSMIT -ve

So we transmit from the DTE and receive on the DCE and vice versa (BINGO).

Remember Routers are regarded as DTEs
=================================

So lets connect 2 switches  together with a CROSSOVER cable ( Function DCE to DCE)

UPLINK to UPLINK

SWITCH = DCE = MDI(X)                           SWITCH = DCE = MDI(X)
RJ45 PIN                                           RJ45 PIN
1  -- RECEIVE +ve -------------------------------- 3  -- TRANSMIT +ve
2  -- RECEIVE -ve --------------------------------- 6  -- TRANSMIT -ve
3  -- TRANSMIT +ve ------------------------------ 1  -- RECEIVE +ve
6  -- TRANSMIT -ve ------------------------------- 2  -- RECEIVE -ve

So we transmit from the DCE and receive on the DCE and vice versa (BINGO).

Regards
Alex

[SJ K]

Hi Alex,

Thanks for the illustration. That actually confirm my thoughts. Uplink to Uplink will be usually using a crossover cable.

But as illustrated by you, you mentioned SWITCH = DCE = MDI(X) whereby the pinouts are

1  -- RECEIVE +ve
2  -- RECEIVE -ve
3  -- TRANSMIT +ve
6  -- TRANSMIT -ve

=======================================

How about on a switch with normal ethernet ports and designated uplink ports ? are all the both/all the ports MDI(X) then ?

http://www.thecomputerpartstore.com/cisco-catalyst-2950-series-wsc2950t48si-1000mbps-uplinks-switch

As again, from the previous website I have given, the uplink does not crossover.
http://duxcw.com/faq/network/uplink.htm

==========

So i am thinking for switch to switch.

MDIX = normal ethernet port

1  -- RECEIVE +ve
2  -- RECEIVE -ve
3  -- TRANSMIT +ve
6  -- TRANSMIT -ve

 

Normal ethernet (MDIX) to  normal ethernet (MDIX) = crossover cable to be used

Uplink port (MDI) to Normal Ethernet port (MDIX) = straight cable to be used

Uplink port (MDI) to Uplink Port (MDI) = crossover = crossover able to be use

Am I right ?
 

Regards,
Noob

[SJ K]

Hi Alex,

Hope to hear from you soon with regards to my last post.

Thanks.

Regards,
Noob

[acampbell · Accepted solution]

Hi Noop.

(Sorry for the delay)

By default on all switches and hubs normal port
are MDI(X) as they are part of the DCE function.

If you physicaly use an uplink port like by pushing a we button etc
you ar turning the port into the MDI state therefore to link to another
switch/hub you use a straight cable :-

Switch UPLINK (MDI -DTE) ----- STRAIGHT ---- Switch NORMAL PORT (MDIX-DCE)

Have a look at this page:-
http://en.wikipedia.org/wiki/Medium-dependent_interface

Hope this helps
Regards
Alex

[SJ K]

Clear as crystal.

Thanks Alex.!