Spanning-tree: changing FWD vs BLK ports

hello, everyone:

i am testing spanning-tree behavior and my books seems to indicate that the
blocking/forwarding state on the root ports are determined in the following
order:

1. port with lowest priority
2. port cost
3. lowest port number, if tie between the above two.

based on the assumption above, i have 2 switches whereby when i change the
port priority on gig 0/48 to a lower value to force it to become the FWD
port, i don't get any movement whatsoever. below is my config from both
routers. although it shows MST, i got the same result when i was running a
single instance STP:

**********First Switch - ROOT**************

DLS1-3560-Mid#sho span mst

###### MST00 vlans mapped: 1-4094
Bridge address 0015.63e3.6b80 priority 32768 (32768 sysid 0)
Root this switch for CST and IST
Configured hello time 2, forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/1 Desg FWD 200000 128.3 P2p
Fa0/2 Desg FWD 200000 128.4 P2p
Fa0/17 Desg FWD 200000 128.19 P2p
Fa0/18 Desg FWD 200000 128.20 P2p
Fa0/33 Desg FWD 200000 128.37 P2p
Fa0/47 Desg FWD 200000 128.51 P2p
Fa0/48 Desg FWD 200000 128.52 P2p

DLS1-3560-Mid#

*****************Second Switch, NON-ROOT****************

DLS2-3560G-Bottom#sho span m
05:01:00: %SYS-5-CONFIG_I: Configured from console by console
DLS2-3560G-Bottom#sho span mst

###### MST00 vlans mapped: 1-4094
Bridge address 0016.9ddd.d700 priority 32768 (32768 sysid 0)
Root address 0015.63e3.6b80 priority 32768 (32768 sysid 0)
            port Gi0/1 path cost 0
IST master address 0015.63e3.6b80 priority 32768 (32768 sysid 0)
                                    path cost 200000 rem hops 19
Operational hello time 2, forward delay 15, max age 20
Configured hello time 2, forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Gi0/1 Root FWD 200000 128.1 P2p
Gi0/2 Altn BLK 200000 128.2 P2p
Gi0/17 Altn BLK 200000 128.17 P2p
Gi0/18 Altn BLK 200000 128.18 P2p
Gi0/33 Altn BLK 200000 128.33 P2p
Gi0/47 Altn BLK 200000 128.47 P2p
Gi0/48 Altn BLK 200000 16.48 P2p

DLS2-3560G-Bottom#
***********************************

what am i missing. i know for sure that when i change the cost to a lower
value, i do get the sought behavior. but i want to make sure i know at least
another way of changing the root port.

thanks in advance

JEP.

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Received on Sat Mar 27 2010 - 16:04:48 ART