Fast aging of trunk MAC ?

I've run into a rare problem where one switch was sending the
default GW traffic to all ports.

        L2net
          |
7200a -- 3550a -- 2950 -- Host

The 7200a is teamed with a 7200b using HSRP, and the 2950 is receiving
all the HSRP MAC traffic because the 3550 is not "learning" the MAC.

It turns out that the 7200 - 3550 connection is a trunk with 2 vlans,
and portfast was not being honoured, so the vlan status was:

#sh spanning-tree vlan 5

MST0
  Spanning tree enabled protocol mstp
  Root ID Priority 16384
             Address 000e.6a97.3100
             Cost 200004
             Port 24 (FastEthernet0/24)
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
             Address 000f.2496.5d00
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- --------
--------------------------------
Fa0/10 Desg FWD 200000 128.10 P2p Edge
Fa0/11 Desg FWD 200000 128.11 P2p Edge
Fa0/12 Desg FWD 200000 128.12 P2p Edge
Fa0/15 Desg FWD 200000 128.15 P2p
Fa0/22 Desg FWD 200000 128.22 P2p Bound(STP)
Fa0/24 Root FWD 200000 128.24 P2p Bound(RSTP) Pre-STD-Rx

Port Fa0/15 is the 7200.
I've set "spanning portfast trunk" in the port and now the MAC is not
fast aged.

Anybody has an idea of why this is/was happening ?

-Carlos

-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net