This article describes a Linux server set up with 2 interfaces (eth0) and (eth1). Each interface has a separate ISP, network details and default gateway. eth0 has two sets of network details on the same interface and so a virtual interface (eth0:0) must be created to handle the second IP.
By default, Linux only allows for one default gateway. Let’s see what happens if we try to use multiple uplinks with 1 default gateway. Assume eth0 is assigned 192.168.1.2/24 and eth1 is assigned 172.16.1.5/16. Let’s say our default gateway is 192.168.1.1 (which of course is found through eth0) but there’s also a 172.16.0.1 gateway on eth1 which we can’t enter as Linux only allows for the one.