Linux Advanced Routing / Two Default Gateways:

http://lartc.org/howto/index.html
https://www.thomas-krenn.com/en/wiki/Two_Default_Gateways_on_One_System
http://www.rjsystems.nl/en/2100-adv-routing.php
https://kindlund.wordpress.com/2007/11/19/configuring-multiple-default-routes-in-linux/
http://www.dfwavc.com/linux_multi_nic-multi_gateway

Add table:

echo -e "10\trt2" >> /etc/iproute2/rt_tables

Add route/rule:

ip route add 1.2.3.0/20 dev eth1 src 1.2.3.172 table rt2
ip route add default via 1.2.3.1 dev eth1 table rt2

ip rule add from 1.2.3.172/32 table rt2
ip rule add to 1.2.3.172/32 table rt2

ip rule add from 1.2.3.173/32 table rt2
ip rule add to 1.2.3.173/32 table rt2

ip route flush cache
ip route list table rt2
ip route show
ip rule show

Testing:

ping -I 1.2.3.172 8.8.8.8

Making it permanent:

debian:

/etc/interfaces
/etc/network/interfaces.d/eth1

post-up ip route add 1.2.3.0/20 dev eth1 src 1.2.3.174 table rt2
post-up ip route add default via 1.2.3.1 dev eth1 table rt2
post-up ip rule add from 1.2.3.174/32 table rt2
post-up ip rule add to 1.2.3.174/32 table rt2

/etc/network/interfaces.d/eth1:0

post-up ip rule add from 1.2.3.176/32 table rt2
post-up ip rule add to 1.2.3.176/32 table rt2


redhat:

echo "1.2.3.0/20 dev eth1 src 1.2.3.172 table rt2" >> /etc/sysconfig/network-scripts/route-eth1
echo "default via 1.2.3.1 dev eth1 table rt2" >> /etc/sysconfig/network-scripts/route-eth1

echo "from 1.2.3.172/32 table rt2" >> /etc/sysconfig/network-scripts/rule-eth1
echo "to 1.2.3.172/32 table rt2" >> /etc/sysconfig/network-scripts/rule-eth1
echo "from 1.2.3.173/32 table rt2" >> /etc/sysconfig/network-scripts/rule-eth1
echo "to 1.2.3.173/32 table rt2" >> /etc/sysconfig/network-scripts/rule-eth1