Use Cases to enable multiple subnet/vlan/network on your machine:
Some times you will need your server to be talking through different networks (VLANS) for some reasons. Example, I need to create a hypervisor (Physical server which holds all my Virtual Machines) with KVM which gives VMs in multiple VLANs. Taking this example to explain this setup:
1. Production VLAN/SUBNET:
2. Dev QA VLAN:
Need to Create the hypervisor which can Give Virtual Machines on both the VLAN/SUBNET. If we simply bring up the hypervisor with two NICs up on both the VLAN, either of the NICs will be working not both. The ideal setup should be as below:
1. Create two bridge interfaces on the active physical interfaces in "/etc/network/interfaces" file
iface lo inet loopback
iface br1 inet static
post-up route add default gw 192.168.75.1 metric 1
pre-down route del default gw 192.168.75.1
iface br2 inet static
post-up route add default gw 192.168.76.1 metric 2
pre-down route del default gw 192.168.76.1
2. Restart the Network #/etc/init.d/networking restart
3. There will be rp_filters enabled for the bridge interfaces. Need to disable that. In this case br0 and br1:
#echo "0" >/proc/sys/net/ipv4/conf/br0/rp_filter
#echo "0" >/proc/sys/net/ipv4/conf/br1/rp_filter
#echo "0" >/proc/sys/net/ipv4/conf/all/rp_filter
This will make sure both the interfaces are active and reachable on the hypervisor.
Make sure the rp_filters are disabled on reboot as well by adding the above commands in some startup files.
You are Done!