I have very strange problem. In a office computer network with about 100 PCs with mixed Windows XP SP3, Windows 7 and some Windows Server 2008 R2, I've decided to upgrade the network from Fast Ethernet to Gigabit by changing the main 100Mbps switch (a TP-Link) with a 1Gbps switch (ZyXEL GS1910-48). There are two other gigabit switches (TP-Link) that I preserved in the network and they are uplinked to the new one.
After this change the network performance instead to be optimized it degraded! A group of the PCs (about 15) use accounting software from Windows Server 2008 as mapped windows drive to load the software and uses TCP connection to the server when accessing and writing accounting data. The performance of that network after the change degraded like 10Mbps network and every operation which was fast before the change become slow.
I checked the flow control of the switch and it is disabled on every port. Also disabled the flow control and QoS of every client machine. The switch port on which is connected the accounting server I noticed the error and rejected packet counters are increasing almost every second. Every ethernet cabling is changed with brand new CAT6. Some of the PCs have 100Mbps cards and other have 1Gbps. When I manually change speed in clients to be 100Mbps FDX is better performance but not as like before the switch change. Any ideas?