mjhjmtu

I need to add a route that won't be deleted after reboot. I read these two ways of doing it :

Add up route add -net 172.X.X.0/24 gw 172.X.X.X dev ethX to the file /etc/network/interfaces

or

Create the file /etc/network/if-up.d/route with:

#!/bin/sh
route add -net 172.X.X.0/24 gw 172.X.X.X dev ethX

and make it executable :

chmod +x /etc/network/if-up.d/route

So I'm confused. What is the best way of doing it?

You mentioned /etc/network/interfaces, so it's a Debian system...

Create a named routing table. As an example, I have used the name, "mgmt," below.

echo '200 mgmt' >> /etc/iproute2/rt_tables

Above, the kernel supports many routing tables and refers to these by unique integers numbered 0-255. A name, mgmt, is also defined for the table.

Below, a look at a default /etc/iproute2/rt_tables follows, showing that some numbers are reserved. The choice in this answer of 200 is arbitrary; one might use any number that is not already in use, 1-252.

#
# reserved values
#
255 local
254 main
253 default
0 unspec
#
# local
#

Below, a Debian 7/8 interfaces file defines eth0 and eth1. eth1 is the 172 network. eth0 could use DHCP as well. 172.16.100.10 is the IP address to assign to eth1. 172.16.100.1 is the IP address of the router.

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The production network interface
auto eth0
allow-hotplug eth0
# iface eth0 inet dhcp 
# Remove the stanzas below if using DHCP.
iface eth0 inet static
 address 10.10.10.140
 netmask 255.255.255.0
 gateway 10.10.10.1

# The management network interface
auto eth1
allow-hotplug eth1
iface eth1 inet static
 address 172.16.100.10
 netmask 255.255.255.0
 post-up ip route add 172.16.100.0/24 dev eth1 src 172.16.100.10 table mgmt
 post-up ip route add default via 172.16.100.1 dev eth1 table mgmt
 post-up ip rule add from 172.16.100.10/32 table mgmt
 post-up ip rule add to 172.16.100.10/32 table mgmt

Reboot or restart networking.

Update - Expounding on EL

I noticed in a comment that you were "wondering for RHEL as well."
In Enterprise Linux ("EL" - RHEL/CentOS/et al), create a named routing table as mentioned, above.

The EL /etc/sysconfig/network file:

NETWORKING=yes
HOSTNAME=host.sld.tld
GATEWAY=10.10.10.1

The EL /etc/sysconfig/network-scripts/ifcfg-eth0 file, using a static configuration (without NetworkManager and not specifying "HWADDR" and "UUID" for the example, below) follows.

DEVICE=eth0
TYPE=Ethernet
ONBOOT=yes
NM_CONTROLLED=no
BOOTPROTOCOL=none
IPADDR=10.10.10.140
NETMASK=255.255.255.0
NETWORK=10.10.10.0
BROADCAST=10.10.10.255

THE EL /etc/sysconfig/network-scripts/ifcfg-eth1 file (without NetworkManager and not specifying "HWADDR" and "UUID" for the example, below) follows.

DEVICE=eth1
TYPE=Ethernet
ONBOOT=yes
NM_CONTROLLED=no
BOOTPROTOCOL=none
IPADDR=172.16.100.10
NETMASK=255.255.255.0
NETWORK=172.16.100.0
BROADCAST=172.16.100.255

The EL /etc/sysconfig/network-scripts/route-eth1 file:

172.16.100.0/24 dev eth1 table mgmt
default via 172.16.100.1 dev eth1 table mgmt

The EL /etc/sysconfig/network-scripts/rule-eth1 file:

from 172.16.100.0/24 lookup mgmt

On Debian based distro you can add a static route permanently as follows:

 echo "up route add -net 172.X.X.X/24 gw 172.X.X.X dev ethX" | sudo tee --append /etc/network/interfaces

On RHEL based distro:

echo "172.X.X.X/24 via 172.X.X.X" | sudo tee --append /etc/sysconfig/network-scripts/route-ethX