Are you using a Router with Classical IP and LAN Emulation (LANE)?

Performance problems occur if a router uses ATM Classical IP (default 9180 byte MTU) and LAN Emulation (default 1500 byte MTU) links simultaneously when a TCP connection is set up using one interface in one direction and the other interface in the opposite direction, TCP is confused about the maximum packet size.

For example, suppose a TCP connection is set up between Host A and Host B, where packets from Host A travel to Host B over the LANE interface and packets from Host B travel to Host A travel over the Classical IP interface. Host A attempts to send a 9180 byte packet that cannot traverse the LANE network to Host B. TCP recovers from this error and retransmits the packet, but a significant performance loss will be noted.

Possible workarounds to improve performance are:

• Adjust the MTU size, if possible, of the Classical IP link to 1500 bytes.

• Depending upon the network topology, adjust the routing table on Host B to ensure that the route back to Host A points to the LANE interface.

This problem is not unique to ATM networks. It may affect any network configuration that has multiple routes with differing MTUs (such as FDDI and Ethernet or Token Ring). The problem is more pronounced with ATM subnets because of the different default MTUs of Classical IP and LANE.