目前的解决方案：
1、开启STP协议
2、将br-lan的与其子网下的hwaddr改成不同的
这两种目前正在测试，效果未知。
============== 2018.07.27 ==============
目前来看，好像管用！
root@OpenWrt:~# ifconfig
br-lan Link encap:Ethernet HWaddr B8:27:EB:9A:7B:7B
inet addr:192.168.100.1 Bcast:192.168.100.255 Mask:255.255.255.0
inet6 addr: fe80::ba27:ebff:fe9a:7b7b/64 Scope:Link
inet6 addr: fd68:a3d7:3827::1/60 Scope:Global
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:696 errors:0 dropped:0 overruns:0 frame:0
TX packets:100 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:152747 (149.1 KiB) TX bytes:14742 (14.3 KiB)
eth0 Link encap:Ethernet HWaddr B8:27:EB:CF:0E:0E
inet addr:192.168.0.123 Bcast:192.168.0.255 Mask:255.255.255.0
inet6 addr: fe80::ba27:ebff:fecf:e0e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:3018 errors:0 dropped:0 overruns:0 frame:0
TX packets:2558 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3862572 (3.6 MiB) TX bytes:268825 (262.5 KiB)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:109 errors:0 dropped:0 overruns:0 frame:0
TX packets:109 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1
RX bytes:10857 (10.6 KiB) TX bytes:10857 (10.6 KiB)
wlan0 Link encap:Ethernet HWaddr B8:27:EB:9A:5B:5B
inet6 addr: fe80::ba27:ebff:fe9a:5b5b/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:721 errors:0 dropped:0 overruns:0 frame:0
TX packets:734 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:154774 (151.1 KiB) TX bytes:188138 (183.7 KiB)
========================= 2018.09.28 ==============================
最新的OpenWrt tag18.06.1已经修复了这个问题。
（其实是通过每次开机随机一个MAC实现的，异曲同工）