So you're seeing low throughput in your mixed gigabit / 100 Mbps LAN, you have a few options to fix it:
- Disable Flow Control in the NICs - This is the lowest-cost approach and should be tried first. But your adapter might not allow you to futz with Flow Control. I took a survey of the Ethernet adapters in my various machines and found that only the Intel PRO/1000 MT Desktop and internal 3C920 (3C905C-TX compatible) Integrated adapter had Flow Control properties.
Two notebooks that had Realtek RTL8139/810x family integrated 10/100 NICs did not show Flow Control properties, even though the chipset does support Flow Control. If my experiments are accurate, however, you only need to worry about diabling flow control in the gigabit adapters anyway.
- Disable Flow Control at the switch - It's unlikely you would have to resort to this, since you should be able to disable Flow Control in your gigabit NICs. But if you can't, then you need to get yourself a "smart" gigabit switch that has Flow Control enable/disable. You can't do this by changing to a different unmanaged gigabit switch, since all of them support 802.3x flow control, which can't be disabled.
But moving up to a "smart" switch might not be as painful a hit in the wallet as you'd think. For example, the unmanaged Linksys SD2008 can be found for as little as $71, while its "smart" SLM2008 cousin is around $100. And in addition to Flow Control, you get other goodies such as VLANs, bandwidth control, port mirroring, link aggregation and more.
- Upgrade to all gigabit NICs -
This is a good solution if you have desktop machines, since PCI NICs can be had for as little as $12. But if your notebooks have only 10/100 Ethernet ports, or you have devices with embedded NICs such as media players or NASes, you won't be able to upgrade.
By the way, you may need to hunt around for your NIC's Flow Control setting. I finally managed to find the PRO/1000 MT Desktop adapter's setting buried as shown in Figure 4.
Figure 4: Flow Control option in Intel PRO/1000 MT Desktop adapter
By contrast, the 100 Mbps NIC's Flow Control setting was one of the top-level settings in the adapter's Advanced Network Properties.