eth0 : 192.168.1.3/24  << provided by dhcp from router
default gateway: 192.168.1.1

eth1 : 172.21.1.10/24   << static ip I added.

no default gateway for eth1.

ip show route:
192.168.1.0/24 dev eth0 ... src 192.168.1.3
172.21.1.0/24 dev eth0 ... src 172.21.1.10
default via 192.168.1.1 dev eth0 ... src 192.168.1.3