MAC Aging Time Behavior

I am working on a port-security deployment and noticed something interesting
to me. I was wondering if anybody else has seen this or can explain this
particular situation. In this particular environment, we have IP phones
directly connected to Cisco 3750-x access-layer ports. PCs are then plugged
into the phones. I am using dynamic secure address learning with the below
configuration:

 switchport port-security maximum 2
 switchport port-security maximum 1 vlan access
 switchport port-security maximum 1 vlan voice
 switchport port-security
 switchport port-security aging time 5
 switchport port-security aging type inactivity

My original thought was to configure an aging time of 5 minutes of
inactivity because aging is disabled by default (set to 0). The
documentation seems to indicate that without setting an aging time,
dynamically learned addresses will simply never age out. That all makes
sense.

Here is the interesting part to me -- If I unplug the PC from the downstream
phone, the dynamically learned secure MAC address is immediately aged out on
the switch. Also, the mac address is aged out of the mac address table
immediately. I am wondering, how does this happen when the device being
disconnected is downstream off another "switch". When I disconnect the PC
from the switch port of the phone, does the phone in fact "signal" to the
upstream switch somehow? If so, how does this happen? I can't find
anything that explains that.

One thought I had was STP TCN, but I am running RSTP on the switch and edge
ports transitioning to down do not count as changes in RSTP. My only other
thought is some sort of magic in CDP but I can't find anything that says
that.

Thanks guys for any feedback!

-- 
Regards,
Joe Astorino
CCIE #24347
Blog: http://astorinonetworks.com
"He not busy being born is busy dying" - Dylan
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