How to configure debain 10 as router?

This is topic talk about my first explore how to configure Linux as router base on Debian 10.

debian@Router:~$ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether ca:76:e7:b0:16:4a brd ff:ff:ff:ff:ff:ff
debian@Router:~$ ip route show
default via 192.168.88.1 dev eth0 
192.168.88.0/24 dev eth0 proto kernel scope link src 192.168.88.83

I just add one more interface Eth1

debian@Router:~$ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether ca:76:e7:b0:16:4a brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether e6:d3:81:e5:b6:46 brd ff:ff:ff:ff:ff:ff

add IP address 192.168.1.1 to interface Eth1

debian@Router:~$ sudo ip addr add 192.168.1.1/24 dev eth1

Show IP address on interface Eth1 after configure

debian@Router:~$ ip addr show eth1
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether e6:d3:81:e5:b6:46 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.1/24 scope global eth1
       valid_lft forever preferred_lft forever

Show link on interface Eth1

debian@Router:~$ ip link show eth1
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN mode DEFAULT group default qlen 1000
    link/ether e6:d3:81:e5:b6:46 brd ff:ff:ff:ff:ff:ff

Open link on interface Eth1

debian@Router:~$ sudo ip link set eth1 up
debian@Router:~$ ip link show eth1
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether e6:d3:81:e5:b6:46 brd ff:ff:ff:ff:ff:ff