I'm running CentOS 6.5. I have 3 servers with quad 1Gb nics. I'm using bonding mode 6 (tried 0 as well) running through a cisco 4948 switch. When only two network cables are attached, no issues. If more than two are connected, the network randomly drops packets, and we are randomly unable to connect to the server. I was fortunately able to determine that the problem went away when one or two interfaces are connected.
DEVICE=bond0 ONBOOT=yes IPADDR=192.168.1.10 NETMASK=255.255.255.0 NETWORK=192.168.1.0 GATEWAY=192.168.1.1 USERCTL=no BOOTPROTO=no BONDING_OPTS="mode=6 miimon=100"
ifcfg-eth0 through 3:
DEVICE=eth0 ONBOOT=yes USERCTL=no MASTER=bond0 SLAVE=yes BOOTPROTO=no
alias netdev-bond0 bonding