Linux – Openwrt two nic on the same vlan instead of alias

Sure, you simply have to enable vlan tagging on the switch and the network adapter and setup both sides with all the vlans you want the computer to see.

The details about how to configure vlan trunking vary depending on what OS, and what you have to do for a specific nic or switch.

Keep in mind that this may be a bit of a security issue. Suppose this is connected to two networks which have some firewalling between them. The computer that spans these two networks becomes an alternate pathway. If an attacker can compromise the system, then they can use the system jumping point to get to the inside hosts.

Since they are different networks, you can just set up Source Based Routing using Policy based routing to route out different interfaces based on the source IP address of the outgoing packet.

For Cisco IOS is is basically the following (I think, untested) (F0/0 is the internal interface, 12.12.12.12, and 13.13.13.13 are your two IP gateways, you have two LANs 192.168.0.0/16 and 10.0.0.0/8):

interface FastEthernet0/0
     ip policy route-map foo-out

route-map foo-out permit 10
 match ip address match-foo-out
 set ip next-hop 12.12.12.12

route-map foo-out permit 11
 match ip address match-foo2-out
 set ip next-hop 13.13.13.13

ip access-list extended match-foo-out
 deny   ip 10.0.0.0 0.255.255.255 any
 permit ip 192.168.0.0 0.0.255.255 any


ip access-list extended match-foo2-out
 deny   ip 192.168.0.0 0.0.255.255 any
 permit ip 10.0.0.0 0.255.255.255 any

If these are just Frame-Relay then set interface instead of next hope would be okay as well.

For your inter-company communication, you wouldn't really have to do anything but change those acls so they deny source and dest where it is the same company, ie: deny ip 192.168.0.0 0.0.255.255 10.0.0.0 0.255.255.255. Also if these are two lan interfaces maybe a different route map for each interface makes more sense or is required, this is just meant to be an example to push you in the right direction, hopefully :-)