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Do gigabit ports have a total throughput of 1 gigabit so that you could be sending 800 mbit and receiving 200 mbit at the same time. Or is it 1000 in and 1000 out?

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3 Answers

up vote 6 down vote accepted

That depends on whether you're running in half duplex (1 gigabit total) or full duplex (1 gigabit each direction). Of course, you'll never hit these theoretical performance limits.

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Hmmm, jumbo frames on an iSCSI network I see around 110MB/sec - that's about the maximum performance. – Mark Henderson♦ Mar 24 '10 at 23:07
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Don't mix-up/confuse MB (bytes) and Mb (bits). Networks normally work in bits while storage works in bytes. Just to make things more interesting... – Peter Mar 24 '10 at 23:11
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Worth mentioning that half-duplex gigabit is pretty much an academic consideration - Many gigabit devices don't even bother to support operation in half-duplex mode. Noted by Intel in one of their white papers: intel.com/network/connectivity/resources/doc_library/… – Chris Thorpe Mar 24 '10 at 23:14
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@Chris Thorpe: +1. Good point. While half-duplex GBe is technically and theorretically possible, I've never actually seen a NIC or switch that supported it or allowed it to be configured. – joeqwerty Mar 24 '10 at 23:19
up vote 5 down vote

It's not quite as simple as that, but yes, GBe is full duplex, so data can flow in both directions simultaneously.

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Correct. The total throughput of a single Gigabit Ethernet Port is 2000Mbps, and thus the total throughput of a (good) Gigabit Ethernet Switch is (no of ports)x2000 (so, a 48-port switch should have a total backplane capacity of 96,000Mbps. Now you know why they're so damn expensive) – Mark Henderson♦ Mar 24 '10 at 23:05
@farseeker: They definitely don't have to have a backplane to support full duplex on all of the ports. Most high end switches will advertise their backplane and it usually isn't full capacity. Thats why there are cheap switches and really expensive ones. Check out this page: cisco.com/en/US/prod/collateral/switches/ps5718/ps7077/… The 48 port switches have 68Gbps backplanes. If you buy a 48port Cisco SB it probably has much less – einstiien Mar 25 '10 at 0:20
Here's one that has only 17Gbps for 48ports: cisco.com/en/US/prod/collateral/switches/ps5718/ps9994/ps10004/… – einstiien Mar 25 '10 at 0:25
@einstiien - those pages pretty much backup what I meant. Expensive switches, or cheap switches. I can see how my comment wasn't particularly clear. Those 24-port switches certainly do have enough bandwidth for all the gigabit ports to be running at full duplex (excluding the 10Gb ports). – Mark Henderson♦ Mar 25 '10 at 1:15
up vote 1 down vote

If the switch backplane supports it, you should be able to get ~1 Gbps inbound AND outbound. The major limiting factor is the switch backplane and if that only supports 1 Gbps, you'll get packet loss, but if it's sufficiently fast, you'll be able to get a higher aggregated throughput through the switch.

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