Eth not going down after cable disconnected?

hi,

I'd like to ask regarding the behavior of eht:

I want to use eth wire connection as primary ISP connection and wireless connection (using wifi dongle, different network) as backup ISP connection.

But when I disconnect the cable physically, the eth0.101 subinterface is sill up, keeping it's IP address. Here's the config:

cat /etc/network/interfaces

# In real world, this is used on the same situations of auto but the difference is that it will wait an event like "plug the cable" on ethernet interfaces.
allow-hotplug eth0
.

.

.

# receive IP-Address from your DSL- or cablemodem
auto eth0.101
# Replace "dhcp" with "manual" in the next line, if you are going to use "pppoeconf eth0.101" to setup pppoe connection
iface eth0.101 inet dhcp
 

after the cable disconnected, no change in status, UP, keeping IP address

eth0.101  Link encap:Ethernet  HWaddr 02:cb:05:83:0f:e3
          inet addr:192.168.100.9  Bcast:192.168.100.15  Mask:255.255.255.240
          inet6 addr: fe80::cb:5ff:fe83:fe3/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:44044 errors:0 dropped:0 overruns:0 frame:0
          TX packets:27769 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:46705726 (44.5 MiB)  TX bytes:2548115 (2.4 MiB)
 

even the routing table does not change:

root@lamobo-r1:~# netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.168.100.1   0.0.0.0         UG        0 0          0 eth0.101
192.168.100.0   0.0.0.0         255.255.255.240 U         0 0          0 eth0.101
192.168.101.0   0.0.0.0         255.255.255.240 U         0 0          0 br0
 

root@lamobo-r1:~# uname -a
Linux lamobo-r1 3.4.112-sun7i #10 SMP PREEMPT Sun Oct 23 15:49:51 CEST 2016 armv7l GNU/Linux
 

System is updated with latest packages.

Had you experienced the same behavior?

something wrong with eth driver?

any hint how to solve?

Thanks in advance.