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New Member

Half and Full Duplex

Hi,

I having problems with perfomance of my network, I discovery that I have device configured for 100 Half Duplex and other configured for 100 Full Duplex in the same HUB, can it be the problem ?

Thanks,

Alex

10 REPLIES
Cisco Employee

Re: Half and Full Duplex

Hubs are half-duplex only.

www.cisco.com/warp/customer/473/46.html

New Member

Re: Half and Full Duplex

It means that I must configure all my NICs connected to a HUB for 100 HalfDuplex and the port on Switch where the hub is connected to auto ?

Thank you for your assistence,

Alex

New Member

Re: Half and Full Duplex

If it is a 100Mb HUB, then all ports must be 100 half duplex. Strongly suggest that you hard set the switch port that connects to the hub to 100Mb half duplex. Auto is not a good idea for switch to hub connection. You can get unexpected results with auto.

New Member

Re: Half and Full Duplex

OK, and how about the devices connected to HUB, all my computers has 10/100 NICs, do I must configure to 100 Half or can I configure to 100 Full ?

Thanks,

Alex

Cisco Employee

Re: Half and Full Duplex

Either auto-negotiate or set the NICs to half-duplex. Hubs to do not work at full-duplex.

New Member

Re: Half and Full Duplex

Get rid of the Hubs and replace with Switches! (If you can) You'll see a great improvement in your network.

New Member

Re: Half and Full Duplex

Alex,

1. Make sure on the Cisco Switch port that the generic 100MB hub connects to, does NOT have 'spanning-tree portfast' enabled on that port interface.

2. Hard config your NICS for 100/half.

3. And yes, replace that hub as soon as budgets permit, with a Cisco Switch.

Note: when you move the workstations to the switch, remember to bring the NIC configurations back to normal auto speed/auto duplex.

New Member

Half and Full Duplex

I have a question in mind..

Which of the following is a characteristic of full-duplex communication?

It is point-to-point or Hub communication is done via full duplex...

Please answer what is the correct answer on the two choices.   

New Member

Re: Half and Full Duplex

Full duplex means you can send and receive data at the same time. With half duplex, you can only perform one at a time; either sending or receiving but not at the same time.

This goes back to the days of the original ethernet bus. When you wanted to send data onto the ethernet "bus", you would have to listen on the wire and wait until it was clear before you could send your data. Hence, CSMA/CD. Carrier Sense "listening", Multi-Access "everyone using the same wire or bus" with collision detection "listening if you data collided with someone else's requireing you to re-transmit". While you were sending, you could not recieve. So, in the old days, all ethernet was half duplex. Hubs simply made the wire, or bus if you prefer, into a multi-port device. All the rules still applied. You still had to listen to all the communications on the bus(hub) and wait until it was quiet to transmit. Every device on a hub had to listen to every other devices traffic and wait until the wire was quiet to transmit. This was called a collision domain.

Keep in mind we're talking about very short periods of time but the concept is similar to a bunch of people in a room. If everyone is talking at once, there is a lot of collisions. If everyone waits until its quiet, they can talk and have thier message heard. Then the next person waits and then they talk.

With switches came some new advantages;

1. The switch could learn where certain devices were and keep this info in an internal table called the FIB or Forwarding Information Base. Some people also call it a SA/DA table (Source Address/Destination Address). This allowed the switch to forward data from a single source (port 1 for example) to another port (port 10 for example) without anyone else having to listen to that particular data conversation. This only applies to unicast traffic. Broadcast traffic was still sent to every port as it was meant for everyone to hear.

2. Switches also have both a send and recieve pair of wires for each port that connect to a backplane in the switch. This allows the switch to both recieve and send traffic to a device at the same time. Therefore, you no longer have to compete for the wire. Everyone can send and recieve without having to worry about collisions.

The one disadvantage to a switch was higher cost. However, today switches are the norm and hubs are almost obsolete.

For part two of your question, I would say its point to point for unicast traffic and hub-like for broadcast traffic. VLANS have changed that quite a bit but for a single vlan, it pretty much holds true.

Hope this info helps. Switches have gone on to become more then just layer 2 devices but the two things listed above still are what make a switch...well...a switch.

Super Bronze

Re: Half and Full Duplex

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Posting

I having problems with perfomance of my network, I discovery that I have device configured for 100 Half Duplex and other configured for 100 Full Duplex in the same HUB, can it be the problem ?

Yes, very much!

As other posters have noted, hubs are half duplex only.

Without additional analysis, and beyond insuring all your hub connections are using half duplex, likly the next best thing to try is to replace your hub with a switch (as also noted by other posters).

With a hub or hubs, all your 100/half hosts within the same collision domain (which can span multiple hubs) share 100 Mbps.  With a switch, hosts connected to their own port get their "own" 100 Mbps if half duplex, or 200 Mbps if full duplex.

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