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27 votes
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What's the point of auto-sensing 10/100 Mbit/s Ethernet?

Some devices could only run at 10 megabit/s, so the device at the other end would autosense the speed to match. If a device that has a maximum speed of 10 Mbit/s is connected to a 10 Mbit/s /...
Ted Quanstrom's user avatar
15 votes
Accepted

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

To understand this you need to understand the historical context. Originally Ethernet used a shared coaxial cable. Only one device could successfully transmit on this at a time. If two devices ...
Peter Green's user avatar
  • 13.3k
15 votes

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

A hub is really just a powered cable that repeats every signal it receives on one interface to all the other interfaces. If two devices transmit at the same time to the receive of the hub interfaces, ...
Ron Maupin's user avatar
  • 99.8k
14 votes

What's the point of auto-sensing 10/100 Mbit/s Ethernet?

Remember 10Mbps came first, then 100Mbps, then 1000Mbps. The advantage of supporting multiple speeds and automatically switching between them is you can upgrade your network gradually without having ...
Peter Green's user avatar
  • 13.3k
11 votes

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

Great question. In full duplex, there is a dedicated channel for traffic from "left to right" and a dedicated channel from traffic from "right to left": Therefore, in full duplex, collisions are ...
Eddie's user avatar
  • 15k
7 votes

1000BASE-T with "no autonegotiation"

Autonegotiation should only be disabled where necessary. Properly compliant 802.3 hardware will send and respond to information in link pulses. Only in rare instances -- i.e. metro-ethernet -- are ...
Ricky's user avatar
  • 32.2k
6 votes

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

With twisted pair and a repeater hub, the hub is not much more than a digital amplifier. For that it senses a carrier from an incoming signal on one port and switches all other ports to output mode. ...
Zac67's user avatar
  • 84.9k
6 votes

The recommended settings for giga ethernet connected to Ethernet port

In reality, the legacy 10 Mbps ethernet interface probably can't negotiate, and it can probably only do half duplex (very few 10 Mbps interfaces can do full duplex). You should let the 1 Gbps ...
Ron Maupin's user avatar
  • 99.8k
5 votes
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The recommended settings for giga ethernet connected to Ethernet port

The very short answer: don't configure anything. Auto negotiation (or the lack thereof as Ron's detailed) works only when it's left alone. Manual settings can very easily cause problems either right ...
Zac67's user avatar
  • 84.9k
5 votes
Accepted

Static speed and duplex/no-negotiate causing input error

If you disable Auto Negotiation (AN) you need to make sure that both sides are configured in exactly the same way. There's isn't any point in doing that manually, actually, so you should have AN ...
Zac67's user avatar
  • 84.9k
5 votes
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On RJ-45/UTP with 2 pairs, (Tx, Rx) which pair will be used in case of half-duplex?

For 10BASE-T and 100BASE-TX, the MDI side (NICs, routers) transmits on pairs 1-2 and receives on 3-6. On the MDI-X side (hubs, switches) the pairs are swapped. In general, concentrators use MDI-X ...
Zac67's user avatar
  • 84.9k
4 votes
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Verifying 802.3 auto negotiation

With gigabit endpoints (1000BASE-T) your injector needs to be gigabit capable as well. Cheap injectors insert power on the pairs unused by 10BASE-T and 100BASE-TX while disrupting the data ...
Zac67's user avatar
  • 84.9k
4 votes

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

Suppose machine A starts sending data to machine B. As the packet begins to be sent, machine C starts sending different data to machine B. There is only one signal path to machine B, so the ...
David Schwartz's user avatar
4 votes

Static speed and duplex/no-negotiate causing input error

By manually setting the speed and duplex on one side with automatic detection on the other side, the side with automatic speed will detect (not negotiate) the correct speed. For the duplex, ...
Ron Maupin's user avatar
  • 99.8k
3 votes

Reliable transfer but unreliable auto-negotiation?

You simply cannot mix shielded and unshielded parts in a link. The shield only works if it continuous end-to-end and properly grounded on both ends. The actual wires in shielded cabling cannot meet ...
Ron Maupin's user avatar
  • 99.8k
3 votes

On RJ-45/UTP with 2 pairs, (Tx, Rx) which pair will be used in case of half-duplex?

Half-duplex mode in Ethernet still uses 2 pairs. The difference to full-duplex is that it only uses one pair at a time. Half-duplex mode with twisted pair cables only exists because of hubs: Reason ...
jpa's user avatar
  • 356
3 votes
Accepted

Capture auto-negotiation ability flags

Auto negotiation is performed on the physical layer - the NIC driver may report what has been negotiated or it may not (only speed). If you want to look deeper into this I see two variants: Use ...
Zac67's user avatar
  • 84.9k
3 votes

Do I need Auto-Negotiation in 1000BASE-X Ethernet if I have a point-to-point link always at 1 Gbps?

The link probably doesn't need auto-negotiation, but you probably should not change the default without a good reason. There is nothing about having it configured that will cause you any problem, and ...
Ron Maupin's user avatar
  • 99.8k
3 votes

How to connect an old network card (10 Mbps) to a new network (1000Mbps)

The router/switch you are plugging into should negotiate with the card. If you are plugging into a managed switch, you will likely be able to hard code the port's speed/duplex. In a Cisco device this ...
stets's user avatar
  • 1,010
3 votes
Accepted

Why is sometimes a good idea to disable auto-negotiation?

With rare exceptions, disabling Auto Negotiation is not a good idea. Auto Negotiation (AN) is mandatory for 1000BASE-T and faster. It should stay enabled generally. Disabling it makes a default node (...
Zac67's user avatar
  • 84.9k
3 votes

Why is sometimes a good idea to disable auto-negotiation?

It's important to understand that the cases where auto-negotiation should be disabled ARE VERY RARE. Many times study material for tests are a bit out of date. Virtually all equipment manufactured in ...
Ron Trunk's user avatar
  • 67.6k
3 votes
Accepted

How is the maximum speed of an ethernet cable detected?

There's Autonegotiation and that's it. Both sides advertise the speeds and modes they support and the best mutually supported mode is selected. The cable is not tested or trained. If the cable quality ...
Zac67's user avatar
  • 84.9k
2 votes
Accepted

Forced full duplex and autonegotiation

When you specifically set the speed and duplex on one side of a link, that side will not negotiate, so there is no negotiation which takes place. The other side can detect, not negotiate, the link ...
Ron Maupin's user avatar
  • 99.8k
2 votes

1000BASE-T with "no autonegotiation"

In all the PHYs that I have worked with, if Auto-Negotiation is disabled and speed is forced to 1000 Mbps, the PHY treats this as if Auto-Negotiation is enabled and the advertisement is limited to ...
claymation's user avatar
2 votes

How to connect an old network card (10 Mbps) to a new network (1000Mbps)

Have you tried simply plugging it in? Many, even most, (but not all) gigabit ports will negotiate down there. If the one you have won't, buy a small switch that will (look for specs like 10/100/...
Ecnerwal's user avatar
  • 2,746
2 votes

The recommended settings for giga ethernet connected to Ethernet port

A) offer only a subset of speeds In cases where speed/duplex negotiation becomes difficult, you may want to consider the following to restrict a switch to offer only a (sub)set of speeds on a given ...
Marc 'netztier' Luethi's user avatar
2 votes
Accepted

Auto negotiation and SFP+

For Ethernet, Auto Negotiation is only defined for twisted-pair and backplane variants (see IEEE 802.3 clauses 28, 37, and 73). Other PHYs can use auto detection to fall back to a link partner's ...
Zac67's user avatar
  • 84.9k
2 votes

cisco port status regarding the auto-negotiation

What we usually call auto-negotiation is the process by which 2 devices agree on the link speed and duplex. This is really fast (almost instant from a human point of view) and, while issue with this ...
JFL's user avatar
  • 19.7k
2 votes

cisco port status regarding the auto-negotiation

Connecting a non-standard 4-wire/2-pair cable (detail from comment) between 1000BASE-T ports does not normally work. You should not use those cables in general, or at least not in any connection with ...
Zac67's user avatar
  • 84.9k
2 votes
Accepted

Negotiate at 10 Gbit/s but gain lower speeds due to issues?

With the notable exception of "Smartrate" ports supporting any subset of 2.5GBASE-T, 5GBASE-T, 10GBASE-T, Autonegotiation does not train the cable. If the Fast Link Pulses used for AN make ...
Zac67's user avatar
  • 84.9k

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