default gateway --the 0.0.0.0/0 route is set with gateway eg.
auto eth0 allow-hotplug eth0 iface eth0 inet static address 192.0.2.222 netmask 255.255.255.0 network 192.0.2.0 broadcast 192.0.2.255 gateway 192.0.2.1
Some other route is set with up and unset with down eg: for the 10.0.0.0/8 route
auto eth0:1 allow-hotplug eth0:1 iface eth0:1 inet static address 10.21.241.34 netmask 255.255.255.128 network 10.21.241.0 broadcast 10.21.241.127 up route add -net 10.0.0.0 netmask 255.0.0.0 gw 10.21.241.69 down route del -net 10.0.0.0 netmask 255.0.0.0 gw 10.21.241.69
An example /etc/network/interfaces
# cat /etc/network/interfaces # This file describes the network interfaces available on your system # and how to activate them. For more information, see interfaces(5). # The loopback network interface auto lo iface lo inet loopback # The primary network interface auto eth0 allow-hotplug eth0 iface eth0 inet static address 192.0.2.222 netmask 255.255.255.0 network 192.0.2.0 broadcast 192.0.2.255 gateway 192.0.2.1 auto eth0:1 allow-hotplug eth0:1 iface eth0:1 inet static address 10.21.241.34 netmask 255.255.255.128 network 10.21.241.0 broadcast 10.21.241.127 up route add -net 10.0.0.0 netmask 255.0.0.0 gw 10.21.241.69 down route del -net 10.0.0.0 netmask 255.0.0.0 gw 10.21.241.69
The above /etc/network/interfaces produces the following routing table
# route -n Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 0.0.0.0 192.0.2.1 0.0.0.0 UG 0 0 0 eth0 10.0.0.0 10.21.241.69 255.0.0.0 UG 0 0 0 eth0 10.21.241.0 0.0.0.0 255.255.255.128 U 0 0 0 eth0 192.0.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
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Persistent Static Routes Linux Debian