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I have a simple 10/100 router at home. Suppose I connect a gigabit switch to one of its ports and several gigabit devices to the switch.

Will the gigabit devices be able to communicate full speed?

Or does the performance degrade to comply with 10/100 on the other side of the switch?

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migrated from Oct 31 '09 at 12:31

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Changed your wording... it's not a subnet. – MikeyB Oct 31 '09 at 3:09 – Izzy Oct 31 '09 at 4:59

4 Answers 4

The GB devices will communicate through the switch at GB speed but any traffic transiting the router will be limited to 10/100.

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Hubs are a thing of the past. All recent devices are switches. Each line runs at full speed. Occasionally there is an autonegotiation problem and a line is set to the wrong speed, but this is not directly related to having other slower devices on the same switch.

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Just want to point out that hubs are still useful for simplified port mirroring. We run some projects here that it is much easier to use a simple hub for than change around the main switch configuration. – DHayes Nov 2 '09 at 16:14

I just set this up at my house two weeks ago. It works fine. Gigabit switch runs at gigabit (as it should) for things that have gigabit nics and 100 for the rest. I have read some places that some gigabit switches will only be as fast as the slowest device connected. I do not know if this is true, but I bought the Netgear GS108 Switch and it does not do that.

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If it were true, it would be a hub. And I'm pretty sure that Gb hubs don't exist (shudder). – MikeyB Oct 31 '09 at 3:55
I've not seen a hub faster than 10/half... – Nevin Williams Apr 11 '13 at 6:22

In most cases attaching a slower device to a switch will not slow down communication over other ports. So you can expect the gigabit capable devices on the gigabit switch to be able to communicate with each other at gigabit speeds.

However there are some cases where bottlenecks can behave differently from what users would expect. That is due to a concept known as back pressure.

When an outgoing port on a switch experiences congestion, the switch can either drop packets or tell the sender to slow down.

If the switch responds to congestion by dropping packets, then TCP congestion control will kick in, and it can be difficult to utilize the full gigabit speed across a LAN. For this reason some vendors have decided to tell the sender to slow down instead. That works great for sending a single TCP stream across a LAN.

But once multiple flows share a link, back pressure can have undesired side effects.

If one of the gigabit capable devices is sending data simultaneously to two devices where one is able to receive at gigabit speeds and the other can only receive at 100Mbit/s, the switch will notice that the 100Mbit/s link cannot keep up and tell the sender to slow down. When the sender slows down, it affects both flows. So instead of sending 900Mbit/s to one receiver and 100Mbit/s to the other, the sender may end up sending only 100Mbit/s to each receiver.

If you link multiple switches together and have a mixture of different speeds, such problems are more likely to occur. But in principle it can happen even if you have just a single switch with all links running at identical speeds, it just takes the right combination of flows to trigger it.

In my experience these problems are very rare though. I have only seen them while stress testing networks. I haven't experienced them during normal usage.

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