Your customer is using large network packets. And by large packets, this means packet size greater than 1514 bytes.
In computer networking, the maximum transmission unit (MTU) of a communications protocol of a layer is the size (in bytes) of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). Standards (Ethernet, for example) can fix the size of an MTU; or systems (such as point-to-point serial links) may decide MTU at connect time.
Step-by-step guide
Enter root shell
Add MTU to the motherboard NIC (eth0)
echo "MTU=65535" >> /etc/sysconfig/network-scripts/ifcfg-eth0
Turn off hardware-assist offloading on the Ethernet interface.
# set the MSS ifconfig eth0 mtu 65535 # turn off TCP offload ethtool -K eth0 tso off # turn off GRE offload ethtool -K eth0 gso off
To make it permanent, edit and insert the above code into ```/etc/rc.local``` and reboot.