uplink ports and actual switches sfp usages (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

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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

[SJ K]

Can't see in the forum.

[Jon Marshall]

q1) and q2) With Cisco switches and routers as far as I know there is no difference between an uplink port and a normal port except with uplinks they are quite often fiber

switch to switch = crossover
switch to router = straight through
router to router = crossover
host to switch = straight through
host to router = crossover

note that with auto-mdix on switches you can use a straight thru and it will work.

There may be other vendors who do use the uplink port differently but I have not come across it with Cisco

q3) no difference uplink port and normal port on Cisco devices.

q4) SFPs are used for fiber connections on uplink ports. They are connectors and you slot then into the switch port.

q5) fiber does not have crossover in the same way although you need to match up the ends of the fiber.

q6) no difference uplink port and normal port on Cisco devices.

q7) you would stack them if they supported stacking but not all switches do

Edit - one thing that can differ with the uplink ports on switches is that they are often guaranteed to be wire speed ie. no oversubscription whereas with normal ports they may or may not be oversubscribed.

Jon

[SJ K]

Hi Jon,

Thanks for replying. Can you elaborate further on

"Edit - one thing that can differ with the uplink ports on switches is that they are often guaranteed to be wire speed ie. no oversubscription whereas with normal ports they may or may not be oversubscribed."
 

What do you mean by "oversubscribe"  and why can't an uplink port be "oversubscribe" ?

Regards,
Noob
 

[Jon Marshall · Accepted solution]

An uplink port doesn't necessarily run at wire speed but it is more likely to.

But this doesn't mean you don't get oversubscription from the switch to other switches.

It's quite a big subject to cover but i'll try and keep it brief.

Internally within a switch there is a crossbar switch fabric which I think we covered before. Think of it of as pathways between ports. On any switch there are a number of ports that are capable of certain speeds.

Put simply if you take the number of ports on the switch and multiply that by the fastest speed the ports can operate at, if that number is greater than the switch fabric then the switch is said to be oversubscribed.

It doesn't mean in practice it will be oversubscribed but the potential is there.

What can happen though with oversubscribed switches is that the uplinks ports are guaranteed not to be oversubscribed ie. they do not need to contend with the other ports on the switch. It's complicated but it is to do with the connectors per port group etc. to the switch fabric.

But even though the uplinks ports can run at wire speed so they are not oversubscribed, if they are uplinks to other switches there may very well be more traffic combined from the other ports on the switch to that uplink port so you still get an oversubscription ratio.

But what would make it a whole lot worse was if your uplink port was oversubscribed within the switch as well.

So there is oversubscription within the switch and this is where we talk about ports running at wire speed or not and then there is oversubscription between different layers of the network eg. access to distribution for example.

Jon

[SJ K]

Hi Jon,

Put simply if you take the number of ports on the switch and multiply that by the fastest speed the ports can operate at, if that number is greater than the switch fabric then the switch is said to be oversubscribed.

Thanks for elaborating. I am seeing oversubscribe as an "assumption" from the vendor that total port speeds combined will not exceed the what the switch fabric can support.

Let's say I have a 24port which can operation at 1000Mbps which in total is 24Gbps, how can the switch fabric support < 24Gbps then ?

Isn't it cheating ?

 

Regards,
Noob

[Jon Marshall · Accepted solution]

No it's not cheating, it's to do with cost etc.

For example the 6500 switch which is one of Cisco's high end switches has a lot of oversubscribed modules that go into the chassis.

It comes down to a number of things but that is why you should always check the specs before purchasing.

And just because it is oversubscribed in theory it doesn't mean you will get oversubscription because all ports need to be transmitting simultaneously at full speed and that doesn't happen that often with end devices.

It's a judgement the network designer has to make.

Jon

[SJ K]

Hi Jon,

Duly noted. Thanks for the elaboration.

Regards,
Noob