Hi Dear VPP

There is a difference between VPP and Kernel's behavior in route
management. For instance, consider that a route like "ip route add
x.x.x.x/x via y.y.y.y dev ethX" is added. It would be deleted when ethx
down performed in Kernel. While The VPP behavior is that forwarding of that
route entry in unicast-ip4-chain will be changed to dpo-drop. Now what will
happen if we change the state of the interface to up again?! defined route
associated with the kernel interface will be disappeared while VPP will
make previously added route lives again. Which behavior is acceptable? and
is it necessary for VPP to operate the same as kernel anyway?

Kernel senario:
given:  ethX interface has ip and is up
when: "ip route add x.x.x.x/x via y.y.y.y dev ethX" and "ifconfig ethX down"
then: any route dose not exist for ethX in "route -n" output.
then: "ifconfig ethX up" and any route dose not exist for ethX in "route
-n" output again.

VPP senario:
given: GigabitEthernetX/0/0 interface has ip and is up
when: "ip route add x.x.x.x/x via y.y.y.y GigabitEthernetX/0/0" and "set
interface state GigabitEthernetX/0/0 down"
then: "show ip fib x.x.x.x/x detail" => forwarding:   unicast-ip4-chain ...
dpo-drop
then: "set interface state GigabitEthernetX/0/0 up" and "show ip fib
x.x.x.x/x detail" =>
forwarding:   unicast-ip4-chain ... arp-ipv4: via y.y.y.y
GigabitEthernetX/0/0
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