Under what circumstances can/will a 10BASE-T device slow down a 100BASE-T or faster network?

Question

Suppose I have a 10BASE-T device, but everything else on my network is 100BASE-T or faster. When I attach the 10Mbit device, one of two things could happen:

1. It could be passed through as 10BASE-T, making whatever it's connected to slow down to 10BASE-T, or
2. It could be converted to a higher speed.

I'm looking for all the plausible examples where #1 could happen; where attaching this 10Mbit device will slow down other traffic. I sorta think it can't happen - that as soon as it comes into contact with other traffic it has to get retimed to be inserted into the higher rate traffic (so no slowdown aside from the extra bits from the connection), and that when it's not contacting other traffic, who cares if it's only 10Mbit?

Basically I'd like a better understanding of any impact of inserting a slow device into a fast ethernet network.

Answer 1

If you plug a 10BASE-T device into a 10/100 switch, just that one device's port will run at 10mbps. All the ports connected to the 100BASE-TX devices will still go at 100mbps. Regardless of whether the switch is doing cut-through or store-and-forward switching, it operates each port at that port's native speed. It's not like the switch has to cycle all the ports to renegotiate the link speed of all ports down to 10/Half just because a 10/Half client was transmitting a broadcast or multicast. That would be nuts.

If a server on 100BASE-TX needs to send a lot of data to a client on 10BASE-T, the sending server could fill up the client's 10mbit link, but the server would still be able to use the other 90% of its own link to transfer data to other devices.

I could dream up a pathologically bad switch design where the switch only has one small pool of frame buffers shared by all ports, and uses a pathologically bad algorithm for choosing which frames to drop when the buffers fill up, where if any one 100mbps device was sending a lot of data to the 10mbps device, it could fill up all the switch's buffers and keep them dominated by that one traffic flow, causing all other traffic flows across the switch to suffer. But again, that's just trying to dream up a worst-case scenario. It doesn't seem likely that anyone would create a switch that bad.

You may have heard of a (misnamed) beast called a "10/100 dual-speed hub". Those devices are like a 10BASE-T hub bridged to a 100BASE-TX hub, with a scheme to share the same set of physical jacks. Think of it like this: When you plug in a device to a port, the port autonegotiates speed. If it negotiates 10mbps, it connects the port to an internal 10BASE-T hub circuit. If it negotiates 100mbps, it connects the port to an internal 100BASE-TX hub circuit. The two internal hub circuits are connected together via a bridge circuit (a bridge is a 2-port switch). But even if your 10/100 network box with all the RJ-45 jacks turns out to be one of these dual-speed hubs and not a true 10/100 switch, the 10BASE-T device still won't slow down the 100BASE-TX devices, because the bridge circuit in between will do the store-and-forward buffering in each direction.

Answer 2

Assuming you're not using hubs, which force everything to operate at a single speed, using a 10 Mb device won't cause a slow-down on your network.

Answer 3

About the only case I can think of would be related to some kind of multicast operation that includes both 100MB devices and the 10MB device. In a some multicast operations the slowest devices will dictate the speed for everyone. In some cases a really slow member will be dropped out of the group.

Multicast is very uncommon, and it seems even very unlikely that you have a 10base-t device that is going to have anything to do with a multicast meant for other devices on your network.

One somewhat common usage is with a product like Ghost you can use multicast to send out a image to all the systems at the exact same time. The image is only transmuted once.

Answer 4

Well this all sounds good in theory, but reality can be a bit more nasty.

I have a belkin F5D8235-4 v2000 Router with 10/100/1000 ports. Internet connection is 50mb/s (over a 1000mb/s WAN port).

All is well with a mix of 100/1000 devices (full 50mbps broadband speed achieved), but when a 10base-T device is attached (directly to router or through a gigabit switch), internet traffic is unable to get beyond 10mbps on ANY device on the entire network!!

The link speeds are still individually negotiated to 100 or 1000 (or so they claim), but something is dragging traffic down to 10Mbps. As soon as the 10base-t device is disconnected or powered off, everything returns to normal.

So, even if you are sure that everything will be fine, test it thoroughly before walking away!