Add static route

Create a file with the interface name

vi /etc/sysconfig/network-scripts/route-<interface name>

Add static routes (the file must contain at least 2 entries, default route for the interface and your static routes)

default <default gateway> dev <interface name>
<network/netmask> via <gateway for route> dev <interface name>

Example

Adding to reach network 192.168.100.0 with mask 255.255.255.0 using a router at 10.0.0.5 (vlan id 15)

vi /etc/sysconfig/network-scripts/route-vlan15

Routes:

default 10.0.0.1 dev vlan15
192.168.100.0/24 via 10.0.0.5 dev vlan15