02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 03)
For whatever reason it seems the r8169 driver that ships and has shipped with most Linux distros for years is still flawed and does not work properly on these cards, causing 100mbit connectivity when it should be 1gbit and even worse, causing random network dropouts. The r8169 driver in Linux kernels is simply not meant to be used on this cards.
You need to download and compile the driver from here:
For Centos Enable the elrepo
Centos 6: rpm -Uvh http://elrepo.org/elrepo-release-6-5.el6.elrepo.noarch.rpm
yum -y install kmod-r8168
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