uplink port

Answered Question
Dec 9th, 2008

hi everbody!

I have few questions about uplink port on a switch.

My understanding is uplink port is usually used to connect to another switch/hub.When using uplink port to connect to another switch uplink port ,we need staight through cable as pins inside the uplink port is rversed,thereby eliminating the need for cross-over cable. Am I correct? If I am correct, it means if we have to connect host?pc to uplink port, we must use cross-over cable.Is it correct?

thanks a lot!

I have this problem too.
0 votes
Correct Answer by JosephDoherty about 5 years 4 months ago

The end-of-sales 2950T-24 has "24 10/100 ports and two fixed 10/100/1000BASE-T uplink ports". Don't know whether ports are just MDIX or auto-MDI/X, but "uplink" in this sense probably has more to do with the fact they support 10/100/1000 while other ports support only 10/100. If auto-MDI/X not supported, you'll likely need a cross-over cable to another switch's port.

Correct Answer by Mark Yeates about 5 years 4 months ago

Sarah,

I think your original question has been answered I though I would add some advice to it. MDIX can be very handy in the field with using either a straight-through or

a crossover cable. I try to use the proper cable if at all possible for the following reasons:

1:) MDIX is a feature and like many other features it is always not guaranteed to work.

2:) In case you have to replace a switch that supports MDIX auto with one that doesn't makes an equipment swap much more difficult. I have learned this the hard way with a failed 3560 to 3550 swap.

HTH,

Mark

Correct Answer by davy.timmermans about 5 years 4 months ago

Hi Sarah,

In general an uplink port or another port is in general the same.

Between 2 switches: cross (uplink-regular port/ uplink-uplink/ regular-regular)

Between host-switch: straight

between router-router: cross

No matter if it's an uplink port or another port on the switch.

On switches you can configure mdix auto in interface config mode. With mdix auto configured you can use the 'wrong' cable between 2 switches(thus straight). The switch will correct the pin settings. You have to note that if you want to use "mdix auto", you have to put autonegotiation on (speed auto; duplex auto), at least on 1 port of the 2 if I'm correct.

But in general it's a good practice to disable mdix auto and use the correct cable.

If you doubt you can paste the link of the page where you found the information, or try to connect 2 switches.

Correct Answer by JosephDoherty about 5 years 4 months ago

Jon, I've seen same as Giuseppe (I think the same as he's describing) on some (old) consumer switches. They might have two physical ports, both really the same port but the pins were different. I've also seen some of these switches where they might have a hardware DIP to reverse the pins on one switch port. All this before auto-mdix and to allow same patch cable as used by clients. (NB: Don't recall seeing this on "Enterprise" grade switches.)

Correct Answer by Giuseppe Larosa about 5 years 4 months ago

Hello Sarah,

this kind of question is becoming old times with the capability to detect what type of port to use: it is called MDIX.

However, your understanding is correct in small switches the uplink ports are thought to connect to another switch using a straight cable.

In the consumer space there are/have been hubs / switches with a double port 1: one to be used for a host and one to be used if you want to connect another hub/switch

Hope to help

Giuseppe

  • 1
  • 2
  • 3
  • 4
  • 5
Average Rating: 5 (5 ratings)
Jon Marshall Tue, 12/09/2008 - 10:29

Sarah

Wrong way round :-)

Switch to switch would be crossover.

PC to switch would be straight thru.

Basically

PC to router, PC to PC - crossover

PC to switch - straight thru

router to router - crossover

router to switch - straight thru

With the introduction of auto-mdix you can actually use a straight thru cable where a crossover would be needed and the switch will automatically switch the pins.

Jon

Correct Answer
Giuseppe Larosa Tue, 12/09/2008 - 10:31

Hello Sarah,

this kind of question is becoming old times with the capability to detect what type of port to use: it is called MDIX.

However, your understanding is correct in small switches the uplink ports are thought to connect to another switch using a straight cable.

In the consumer space there are/have been hubs / switches with a double port 1: one to be used for a host and one to be used if you want to connect another hub/switch

Hope to help

Giuseppe

Jon Marshall Tue, 12/09/2008 - 10:36

Giuseppe

"However, your understanding is correct in small switches the uplink ports are thought to connect to another switch using a straight cable."

Are you sure about this ? We seem to be giving Sarah different answers. I have never connected 2 switches together without using a crossover (before the advent of auto-mdix that is).

Jon

Correct Answer
JosephDoherty Tue, 12/09/2008 - 10:41

Jon, I've seen same as Giuseppe (I think the same as he's describing) on some (old) consumer switches. They might have two physical ports, both really the same port but the pins were different. I've also seen some of these switches where they might have a hardware DIP to reverse the pins on one switch port. All this before auto-mdix and to allow same patch cable as used by clients. (NB: Don't recall seeing this on "Enterprise" grade switches.)

Jon Marshall Tue, 12/09/2008 - 10:45

Joseph

I understand that there may be some old switches that allow this but take any cisco switch such as 3550/4500/6500 and if you wanted to uplink them using a Cat5 cable it would have to be a crossover and before auto-mdix came along i would say the vast majority of switches, at least Cisco, needed crossover connections.

So i'm not doubting there are some switches where you don't i just didn't want Sarah to get the impression that straight-thru was the common choice because i don't think it was.

Edit - Sorry Joseph, didn't want to come across so strongly it was just the thought of all those countless hours spent looking for a damn crossover cable ...

Jon

JosephDoherty Tue, 12/09/2008 - 11:03

Jon, agree about Cisco switches such as 3550/4500/6500, which is why I made the distinction between consumer switches and Enterprise switches. Also perhaps why Giuseppe wrote "small switches" and "consumer space there are/have been hubs / switches", which I think would exclude at least the 4500 and 6500 and likely the 3550 too. But your point is valid for the "upscale" switches. I.e., you might bump into this feature, dedicated special function "uplink ports", or not. (NB: also the latest consumer switches tend to now have auto-MDX.)

[edit]

"Sorry Joseph, didn't want to come across so strongly it was just the thought of all those countless hours spent looking for a damn crossover cable ... "

No doubt, which I probably why this feature was on the consumer or end-user level equipment, and not on the equipment used by professionals. ;)

Jon Marshall Tue, 12/09/2008 - 11:48

"No doubt, which I probably why this feature was on the consumer or end-user level equipment, and not on the equipment used by professionals. ;)"

Hmmm, now that could be taken any number of ways :-)

Jon

sarahr202 Tue, 12/09/2008 - 11:53

thanks for all of you for replying me !

I understand that if two switches need to be connected by regular ports, we use cross-over cable.

I was reading about up-link ports and how the pins inside the up-link are port changed .

for example if I have two cisco 2950T switches,these two switches have two 10/1000 base-T uplink ports.

Now

1)if I connect them by using up-link ports on both switches, I can not use the straight through cable right? i must use cross over cable?

2) if i connect one switch's uplink port to other switch's regular port, i can use straight through cable. Am i right?

For Switches with no mdix feature,but come with up-link ports , can I safely assume that:

1) use cross over cable if you are using up-link ports on both switches.

2) use straight- thorough cable if you are using one up-link port on one switch to connect to another switch's regular port.

3) use cross-over cable to connect uplink port to PC.( usually we use straight- through cable to connect pc to switch's regular port, since pins are changed inside up-link port, my understanding is we can't use the straight-through cable)

thanks a lot!

JosephDoherty Tue, 12/09/2008 - 12:12

Sarah, the trouble with "up-link" port, its one of those terms that has shades of meaning, the most precise might be a port that is linking to a device up the hiearchy. Just as you might use a "regular" port to be the "downlink" to another switch, the "uplink" on that switch could also be a "regular" port. Also, an "uplink" port might actually be used as a "downlink" port. Some switches and hubs having "special" ports that might be used for uplinks or downlinks. Besides perhaps the pin wiring of these ports being different, they might offer some additional feature not available on all the other ports, perhaps better QoS/CoS or higher bandwidth.

The issure you're dealing with, when auto-MDI/MDIX isn't supported on the port, is the correct matching of the cable, straight vs. cross-over, and MDI vs. MDIX ports. This Wiki article, http://en.wikipedia.org/wiki/Medium_dependent_interface, might help explain.

In short, MDI connects to MDIX (e.g. host to switch) with straight through cable (since one of the ports reverses pins), same port types MDI to MDI (e.g. host to host) or MDIX to MDIX (e.g. switch to switch) would require a cross over cable to reverse the transmit/receive pins.

sarahr202 Tue, 12/09/2008 - 13:13

thanks for your reply!

for example cisco 2950 T has two designated uplink ports.Should i assume pins inside the ports are reversed, requiring straigh through cable to connect to another switch'regular port

assuming mdix feature is not in use ?

davy.timmermans Tue, 12/09/2008 - 13:21

Normally you should use a cross-over.

Assuming pins are reversed as you said: IF they are reversed in both uplink ports, you still should use a cross (is my first thought)

1 <---> 1 normal

-1 <----> -1 'pin reversed'

If you understand what I mean.

But normally it will be a normal cross and no pins are reversed. Uplinkports differ sometimes from regular ports (speed, fiber,...) but not from pinlay-out

Can you plz give your references (URL)

Jon Marshall Tue, 12/09/2008 - 14:30

Sarah

Thanks for that link. I can see what you mean by an uplink port now. I must admit i refer to an uplink port as any port that is used to connect a switch to a switch which is perhaps where some of the confusion comes in.

Just be aware that unlike some of these home type switches if the Cisco switch doesn't support auto-mdix then there is a very good chance you will need to use a crossover cable.

Jon

Giuseppe Larosa Tue, 12/09/2008 - 13:29

hello Sarah, Jon and Joseph

a) in my previous post I was referring to consumer devices that you can find on shelf in a supermarket (I have one at home) not to Cisco regular series where I still use the old rules

b) Another thought about uplink ports in devices like 2950T or similar is that the switching fabric is built in a way that traffic is supposed to flow between user ports and uplink ports. This is reasonable and it was a result of performance tests made by collegues with traffic generator.

So the uplink ports have also this meaning: they are designed to carry more traffic.

Hope to help

Giuseppe

Correct Answer
JosephDoherty Tue, 12/09/2008 - 13:42

The end-of-sales 2950T-24 has "24 10/100 ports and two fixed 10/100/1000BASE-T uplink ports". Don't know whether ports are just MDIX or auto-MDI/X, but "uplink" in this sense probably has more to do with the fact they support 10/100/1000 while other ports support only 10/100. If auto-MDI/X not supported, you'll likely need a cross-over cable to another switch's port.

Correct Answer
davy.timmermans Tue, 12/09/2008 - 12:25

Hi Sarah,

In general an uplink port or another port is in general the same.

Between 2 switches: cross (uplink-regular port/ uplink-uplink/ regular-regular)

Between host-switch: straight

between router-router: cross

No matter if it's an uplink port or another port on the switch.

On switches you can configure mdix auto in interface config mode. With mdix auto configured you can use the 'wrong' cable between 2 switches(thus straight). The switch will correct the pin settings. You have to note that if you want to use "mdix auto", you have to put autonegotiation on (speed auto; duplex auto), at least on 1 port of the 2 if I'm correct.

But in general it's a good practice to disable mdix auto and use the correct cable.

If you doubt you can paste the link of the page where you found the information, or try to connect 2 switches.

Correct Answer
Mark Yeates Tue, 12/09/2008 - 13:17

Sarah,

I think your original question has been answered I though I would add some advice to it. MDIX can be very handy in the field with using either a straight-through or

a crossover cable. I try to use the proper cable if at all possible for the following reasons:

1:) MDIX is a feature and like many other features it is always not guaranteed to work.

2:) In case you have to replace a switch that supports MDIX auto with one that doesn't makes an equipment swap much more difficult. I have learned this the hard way with a failed 3560 to 3550 swap.

HTH,

Mark

Actions

Login or Register to take actions

This Discussion

Posted December 9, 2008 at 10:21 AM
Stats:
Replies:17 Avg. Rating:5
Views:1421 Votes:0
Shares:0
Tags: No tags.

Discussions Leaderboard

Rank Username Points
1 15,007
2 8,155
3 7,730
4 7,083
5 6,742
Rank Username Points
140
73
70
64
45