Centos – How to add interface alias on RHEL6 or CentOS6 with NetworkManager

A couple of things,

1. you didn't bind your bridge to a device
2. Your still attempting to use you Ethernet device normally

try this: -

# vim /etc/network/interfaces

#The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet manual      # set pass through and use br0 for networking

# Bridge setup
allow-hotplug br0
auto br0 
iface br0 inet static
  address 192.168.0.1
  netmask 255.255.255.0
  network 192.168.0.0
  broadcast 192.168.0.255
  gateway 192.168.0.254    # make sure this gateway address is correct
  bridge_ports eth0        # bind to device here
  bridge_stp off 
  bridge_fd 0
  bridge_maxwait 0

First of all, I think you have too many information (and wrong information) in the ifcfg-eth?:? files:

1) I don't think you need/should specify the GATEWAY in all of them, at least to avoid confusions.

2) BOOTPROTO should probably not exist and even if it should, it has an incorrect value (it can be "none", "bootp" or "dhcp").

3) ONBOOT should never be used for alias interfaces and I've seen a lot of trouble caused by this before. It should always be ONPARENT.

4) I think you should always specify the default gateway in /etc/sysconfig/network rather than interface definitions. But that may probably be a personal opinion without a real proof that it is better, just that it sounds more logical to me.

5) The VLAN20 thing that you see in NetworkManager is there from the NAME option, so specify it in your config files as it may be missing I think.