Hi guys,
What's the effect of going 100/Duplex vs 10/Half-duplex in a broadcast environment for DHCP? There are about 10 nodes booting up at an instance.
Advantages and disadvantages... theories and experience please....
Also, when is the right envrionment to use half and full duplex?
Regards,
chobo
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Ok.. There are more than 200 motherboards booting up at almost the same time in a production environment. The existing solution opts for 10/half duplex. Theoretically, I think 100/full duplex is more sustainable, but historically, it was tested that 100/full duplex poses network problem; This is what I've heard from engineers.
To my knowledge, 100/full duplex is good for a low latency environment (Ethernet) rather than a 10/half duplex.
A good example would be the 802.11 wireless, whereby the connection drops from 54Mbps to 22Mbps to 11Mbps to 5.5Mbps, e.t.c.
I hope this doesnt sound like a homework question anymore...
Wired Ethernet is entirely different than wireless - you cannot compare them at the layer 1 & 2 levels. There is A LOT of extra protocol overhead (or under ...) for Wireless Ethernet.
In a full duplex, switched environment - you should NOT have ANY collisions. This the beauty of using full duplex - each station and port can send and receive at the same time. This is because of buffers on the switch capable of receiving traffic on all ports and using seperate circuitry to send on all ports if necessary.
The issue with mixing Autonegotiate with 100 Full is that when you use 100 Full, it disables collision detection circuitry - so the 100 Full side thinks it can send whenever it wants to and DOES NOT listen to the wire first to make sure its clear.
Ideally, IMHO, you should make your entire environment autonegotiate (except for servers) - so that your users can plug in laptops and get optimatl performance and take them home and plug into their SOHO gear and not have any layer1 or 2 issues at home.
>it was tested that 100/full duplex poses network problem;
Not if it was set up correctly.
Here's the thing . . .
Half-duplex expects collisions and uses mechanisms to listen for and to avoid them. Half-duplex is required in a HUB environment where everyone shares the same media in the same collision domain.
Switched Full-duplex cannot have collisions because there is only one host connected to the switchport. Each switchport is its own collision domain.
If you have a switch that is capable of 10/100, then it is by definition full-duplex. All switchports are capable of full or half duplex, 10 or 100Mb. Most switches come out of the box set to auto negotiate both speed and duplex. Most modern network cards in network hosts are capable of auto negotiating the proper speed and duplex. With both the switch and the host set for auto, they both negotiate and usually will settle on 100/full-duplex and both ends are happy happy.
Now many systems "engineers" will suggest that users hard-set their network cards on hosts to 100/full-duplex. WRONG!
What happens then is that the switchport will send a negotiation packet to the host. Host will not respond (negotiation is turned off). Switches have some intelligence and will think "hmmm... this must be a hub connected because hubs don't negotiate, and all hubs are half-duplex" so therfore, it will set its port for half-duplex. Now you have a situation where the host thinks it is full-duplex but the switch is only running at half-duplex. Now you have a duplex mismatch which is the #1 problem for network throughput on any switched network. Now you get collisions on the switched link where they are not supposed to be and the NIC is set to ignore them and won't back off. Now you have performance issues. All because someone a long time ago thought it was a good idea to go ahead and set everyone to 100-full and upgraded the switches that came out of the box set for auto.
You also have an issue with cable plant quality. If your cable plant is not 100% certified CAT5, then you can have speed issues where you can only connect at 10Mb even though both the switchport and the host are capable of 100Mb.
There is never a good reason to run 10M-half-duplex in a switched world, unless specific types of devices are not capable. Then you can manually set the switchport to 10/half to match the host inabilty to negotiate.
Agree with pseudocyber - AUTO everywhere.
Brilliant pseudocyber and lrmoore,
Duplex = no collision domain as each switchport is a collision domain itself! TRUE
Half = collision detection mechanism and should be used in hubs and no point using it in a switch'ed' environment.
The problem right now is with broadcast, what is the most optimum for broadcast settings? full or half with 100 or 10?
I did a packet sniff and these are some of the suspecious results on the Ethereal.
No. = 9
Time = 0.082403
Source = 3com_05:6d:ea
Destination = Broadcast
Protocol = ARP
Info = Who has 10.10.7.25? Gratuituous Arp
This packet is a client. Not the server... I see alot of this kind of arp... Can I know whether this is a problem. The DORA has arleady completed but why is it looking for 10.10.7.25. It is not calling the DHCP server & application server. The DHCP server and application server are on the same physical machine and it has an ip of 10.10.7.2
Regards,
chobo
Duplex settings have nothing to do with broadcasts. VLANs create broadcast domains and can control broadcasts.
You'll always see some gratuitous arps on the network. Unless you're seeing thousands of them I wouldn't worry about it.
http://wiki.ethereal.com/G
I agree with lrmoore.
Expand a bit:
vlans create layer 2 boundries - broadcast domains. IP subnets and routers create layer 3 boundries - layer 3 broadcasts.
Ethernet can broadcast with all 1's represented by F's as in FF.FF.FF.FF.FF.FF. IP Broadcast would be all 1's as well, but can change depending on the size and ... definition/scope/design ... can't think of a good word - of your IP subnet.
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by: grsteedPosted on 2007-01-21 at 21:30:58ID: 18363007
The biggest issue it making sure that both ends of the data link match (PC to hub/Switch, Switch to Router). It they are not matched, you can encounter errors and slow performance on that link, especially if it's a server link or a PC doing file transfers.
As a rule, you want to set both ends for the maximum it is capable of and Full duplex is preferred over Half, again if both ends can handle it. If you have PC's connected to hubs, you are stuck with half duplex. In a switch environment you can and should use Full duplex and the speed whould be at least 100. (10Mbps/Full was only around a short time before 100Mbps/Full came along) In a half duplex environment if 10 stations are trying to send to the DHCP server, they will collide, and start to back off for random amounts of time until each request goes through. I guess in an extreme case, it may affect the DHCP request process, but I haven't seen it.
Here's a good link for troubleshooting speed/duplex issues on Cisco equipment but is good info on the causes and effects of those settings.
http://www.cisco.com/warp/
Cheers,
Gary