We're running CentOS 7.1 and have systems configured with dual NICs setup as a single bonded interface. When this bond is configured to use any of bond modes 1 5, or 6, we are seeing a substantial number of dropped packets. This is most easily observed using the ifconfig command:

bond0: flags=5187<UP,BROADCAST,RUNNING,MASTER,MULTICAST> mtu 1500
inet6 fe80::5054:ff:fe3b:c2a6 prefixlen 64 scopeid 0x20<link>
ether 52:54:00:3b:c2:a6 txqueuelen 0 (Ethernet)
RX packets 29449 bytes 1929707 (1.8 MiB)
RX errors 0 dropped 14924 overruns 0 frame 0
TX packets 516 bytes 40394 (39.4 KiB)

Naturally, we expect to see some dropped packets, but we're seeing rates of 20% and more (the example above is 50%). That's well beyond what should be occurring with normal network traffic. Bond modes 2, 3, and 4 do not exhibit this kind of packet loss, but we do not want to use any of these. Our preference is bond mode 6, but bond mode 5 is acceptable as well. We also don't see any dropped packets if the bond has a single member in it, so it clearly has something to do with how the system is piping traffic through the dual NICs.

Has anyone seen this level of dropped packets using a bonded interface in CentOS 7? We did not see this occur in CentOS 6.5. Our bond is setup using the following ifcfg-bond0 file:

DEVICE=bond0
BOOTPROTO=none
ONBOOT=yes
NM_CONTROLLED=no
IPV6INIT=no
USERCTL=no
TYPE=Bond
BONDING_MASTER=yes
BONDING_OPTS="mode=6 miimon=100 updelay=12000 downdelay=0"

This is a pretty standard bond configuration.

Are the drivers open source or proprietary for the NICs? If they are opensource you might want to audit the kernel changelog to see if there have been any changes that might impact bonding. And/or do a similar audit of the bonding code although unlikely.

Also check your routes.

We're using the stock drivers provided by CentOS 7.1. The routes we're using are correct. They are the same as we used with our CentOS 6.5 based product, and we also used the stock drivers there as well without seeing packet loss. Clearly CentOS 7 introduced this issue. It could be the nic drivers, or perhaps some in the bonding software.