Gigabit Switch Is Slow In One Direction, Fast In Opposite Direction

Question

I have a desktop and a laptop connected by a network switch. Both have Gigabit network cards. My old (Netgear) Fast Ethernet switch gives equal throughput in both directions for file copying operations - 8 MBps. After upgrading to a Gigabit switch (Belkin and later Digisol), file copy operations are fast in only one direction (34 MBps) and slow (less than 1 MBps) in the other direction. In other words, the Gigabit switch can be slower than the Fast Ethernet switch in one direction. If I directly connect the two computers using a crossover cable, the throughput is over 34 MB. The problem is the same in Windows XP and Ubuntu 10.10.

How can I get the Gigabit switches provide namesake speeds in both directions?

A google search reveals that several people have had the same problem but none of them have had a solution yet.

Summary: The Cat 5e+ cables are new. The two Gigabit switches that were used are new. Both computers have Gigabit cards. Gigabit speeds are possible only in one direction. In the other direction, speeds are slower than Fast Ethernet.

Update (12-Nov-2012): I tried the router with another laptop (a new one) and it did not suffer the same problem. So, this is probably an issue with the LAN port of the 4-year-old laptop.

Answer 1

How can I get the Gigabit switches provide namesake speeds in both directions?

You have not provided any information that indicates that either Gigabit switch may be at fault.

The Cat 5e+ cables are new.

Unfortunately this really does not mean much. Especially with cables packaged with some network equipment that are all scrunched up into tiny wads of copper & plastic and violate the minimum bend-radius of UTP cable.

If I directly connect the two computers using a crossover cable, the throughput is over 34 MB.

This is salient information, as it seems to validate both PCs and their NICs as functioning Gigabit equipment. We can use this configuration to test your other equipment.

Most NICs do not support auto-MDI/MDIX (i.e. doesn't care if you use a straight-through or crossover cable, the port will sort it out for you). So you cannot simply replace the crossover cable in your ad-hoc Gigabit network with one of your straight-through patch cables for a simple single component replacement test. You will have to introduce two unknowns at once: the cable-under-test and a RJ-45 coupler. For now we will assume that a simple coupler is reliable and functional for Gigabit Ethernet.

Disconnect the crossover cable at one PC, and attach the coupler plus cable-under-test into the Ethernet link. Run your copy-file test in both directions. If you get the same 35MB/sec transfer speed as without the cable-under-test, then you have verified that you have a good cable (and coupler) for Gigabit. Otherwise the cable would appear to be defective (if the coupler has been validated).

Since most switches do support auto-MDI/MDIX, and unless your switch(es) do not, you may only have to validate one of your "new" (or otherwise) cables in your collection. You should be able to use the proven crossover cable as one of the two cables you need to connect the Gigabit switch to your two PCs.

With two proven-good-for-Gigabit cables, you can then introduce a Gigabit switch into the mix. Note that we are trying to introduce only one unknown at a time into a configuration of known performance.

When performing these tests, you should also inspect the NIC and switch port LEDs for any unexpected conditions (e.g. not a Gigabit link). You should also try to confirm the Ethernet link attributes at each PC for full duplex (and not half duplex) connection at 1000BaseT.