===> We start with re1 UN-PLUGGED:

root@fbsd-router:~ # ifconfig wlan0
wlan0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
        ether 04:f0:21:17:3b:d7
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
        media: IEEE 802.11 Wireless Ethernet autoselect mode 11ng <hostap>
        status: running
        ssid tutu channel 6 (2437 MHz 11g ht/40+) bssid 04:f0:21:17:3b:d7
        country US ecm authmode OPEN privacy OFF txpower 27 scanvalid 60
        protmode CTS ampdulimit 64k ampdudensity 8 shortgi wme burst
        dtimperiod 1 -dfs
        groups: wlan
root@fbsd-router:~ # ifconfig re1
re1: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=82099<RXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,WOL_MAGIC,LINKSTATE>
        ether 00:0d:b9:3c:ae:25
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
        media: Ethernet autoselect (none)
        status: no carrier
root@fbsd-router:~ # ifconfig bridge0
bridge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        ether 02:6b:c0:de:b8:00
        inet 1.1.1.1 netmask 0xffffff00 broadcast 1.1.1.255
        nd6 options=9<PERFORMNUD,IFDISABLED>
        groups: bridge
        id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
        maxage 20 holdcnt 6 proto rstp maxaddr 2000 timeout 1200
        root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0
        member: re1 flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
                ifmaxaddr 0 port 2 priority 128 path cost 55
        member: wlan0 flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
                ifmaxaddr 0 port 5 priority 128 path cost 33333

root@fbsd-router:~ # dtrace -s bridge.d
dtrace: script 'bridge.d' matched 3 probes

===> Now I plug re1:
CPU     ID                    FUNCTION:NAME
  1  16563       do_link_state_change:entry
              kernel`taskqueue_run_locked+0x13b
              kernel`taskqueue_run+0xaf
              kernel`intr_event_execute_handlers+0x1b6
              kernel`ithread_loop+0xa6
              kernel`fork_exit+0x9c
              kernel`0xffffffff80f0dc3e
linkstate for bridge0: UP -> DOWN

====> Now I unplug re1:

  1  16563       do_link_state_change:entry
              kernel`taskqueue_run_locked+0x13b
              kernel`taskqueue_run+0xaf
              kernel`intr_event_execute_handlers+0x1b6
              kernel`ithread_loop+0xa6
              kernel`fork_exit+0x9c
              kernel`0xffffffff80f0dc3e
linkstate for bridge0: DOWN -> DOWN

=====> Now I plug re1 again:

  1  16563       do_link_state_change:entry
              kernel`taskqueue_run_locked+0x13b
              kernel`taskqueue_run+0xaf
              kernel`intr_event_execute_handlers+0x1b6
              kernel`ithread_loop+0xa6
              kernel`fork_exit+0x9c
              kernel`0xffffffff80f0dc3e
linkstate for bridge0: UP -> DOWN

==> How bridge0, which was in "DOWN" state previously, can change from "UP" to "DOWN" ?