As I already have explaned at HOWTO MTU Size for Different Network Topology you can loose a lot of performance in your mixed network if it is not configured. To do that you have to edit the Registry again. First of all you should try to set EnablePMTUDiscovery. WARNING: Using the Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. I can not guarantee that any problems resulting from the use of the Registry Editor can be solved. Use this tool at your own risk. Run regedt32 and edit the registry as described below. EnablePMTUBHDetect Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters Value Type: REG_DWORD - Boolean Valid Range: 0,1 (False, True) Default: 0 (False) Description: Setting this parameter to 1 (True) causes TCP to try to detect "Black Hole" routers while doing Path MTU Discovery. A "Black Hole" router does not return ICMP Destination Unreachable messages when it needs to fragment an IP datagram with the Don't Fragment bit set. TCP depends on receiving these messages to perform Path MTU Discovery. With this feature enabled, TCP will try to send segments without the Don't Fragment bit set if several retransmissions of a segment go unacknowledged. If the segment is acknowledged as a result, the MSS will be decreased and the Don't Fragment bit will be set in future packets on the connection. Enabling black hole detection increases the maximum number of retransmissions performed for a given segment. EnablePMTUDiscovery Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters Value Type: REG_DWORD - Boolean Valid Range: 0,1 (False, True) Default: 1 (True) Description: Setting this parameter to 1 (True) causes TCP to attempt to discover the Maximum Transmission Unit (MTU or largest packet size) over the path to a remote host. By discovering the Path MTU and limiting TCP segments to this size, TCP can eliminate fragmentation at routers along the path that connect networks with different MTUs. Fragmentation adversely affects TCP throughput and network congestion. Setting this parameter to 0 causes an MTU of 576 bytes to be used for all connections that are not to machines on the local subnet. ForwardBufferMemory Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters Value Type: REG_DWORD - Number of bytes Valid Range: - Default: 74240 (enough for fifty 1480-byte packets, rounded to a multiple of 256) Description: This parameter determines how much memory IP allocates to store packet data in the router packet queue. When this buffer space is filled, the router begins discarding packets at random from its queue. Packet queue data buffers are 256 bytes in length, so the value of this parameter should be a multiple of 256. Multiple buffers are chained together for larger packets. The IP header for a packet is stored separately. This parameter is ignored and no buffers are allocated if the IP router is not enabled. MTU Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\\Parameters\Tcpip Value Type: REG_DWORD Number Valid Range: 68 - Default: 0xFFFFFFFF Description: This parameter overrides the default Maximum Transmission Unit (MTU) for a network interface. The MTU is the maximum packet size in bytes that the transport will transmit over the underlying network. The size includes the transport header. Note that an IP datagram may span multiple packets. Values larger than the default for the underlying network will result in the transport using the network default MTU. Values smaller than 68 will result in the tranport using an MTU of 68.