The devices are Cisco 4506 (ports G2/42 and G2/44) and a stack of two 3750s (ports G1/0/1 and G2/0/1).
CDP from 3750s:
4506-Root
Gig 2/0/1 175 R S I WS-C4506 Gig 2/42
3750Stack-NonRoot
Gig 1/0/1 175 R S I WS-C4506 Gig 2/44
It's driving me crazy why STP blocks ports which it does. Please look at the output:
VLAN0002 is executing the rstp compatible Spanning Tree protocol
Bridge Identifier has priority 32768, sysid 2, address 0064.4040.9000
Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6
Current root has priority 4098, address 000b.befd.ac00
Root port is 1 (GigabitEthernet1/0/1), cost of root path is 4
Topology change flag not set, detected flag not set
Number of topology changes 3246 last change occurred 01:01:46 ago
from GigabitEthernet1/0/1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 300
Port 1 (GigabitEthernet1/0/1) of VLAN0002 is root forwarding
Port path cost 4, Port priority 128, Port Identifier 128.1.
Designated root has priority 4098, address 000b.befd.ac00
Designated bridge has priority 4098, address 000b.befd.ac00
Designated port id is 64.108, designated path cost 0
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 5
Link type is point-to-point
Loop guard is enabled by default on the port
BPDU: sent 12, received 379558
Port 55 (GigabitEthernet2/0/1) of VLAN0002 is alternate blocking
Port path cost 4, Port priority 80, Port Identifier 80.55.
Designated root has priority 4098, address 000b.befd.ac00
Designated bridge has priority 4098, address 000b.befd.ac00
Designated port id is 128.106, designated path cost 0
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point
Loop guard is enabled by default on the port
BPDU: sent 3, received 379573
-----
VLAN0020 is executing the rstp compatible Spanning Tree protocol
Bridge Identifier has priority 32768, sysid 20, address 0064.4040.9000
Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6
Current root has priority 4116, address 000b.befd.ac00
Root port is 1 (GigabitEthernet1/0/1), cost of root path is 4
Topology change flag not set, detected flag not set
Number of topology changes 3244 last change occurred 01:02:51 ago
from GigabitEthernet1/0/1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 300
Port 1 (GigabitEthernet1/0/1) of VLAN0020 is root forwarding
Port path cost 4, Port priority 128, Port Identifier 128.1.
Designated root has priority 4116, address 000b.befd.ac00
Designated bridge has priority 4116, address 000b.befd.ac00
Designated port id is 128.108, designated path cost 0
Timers: message age 15, forward delay 0, hold 0
Number of transitions to forwarding state: 3
Link type is point-to-point
Loop guard is enabled by default on the port
BPDU: sent 13, received 379589
Port 55 (GigabitEthernet2/0/1) of VLAN0020 is alternate blocking
Port path cost 4, Port priority 128, Port Identifier 128.55.
Designated root has priority 4116, address 000b.befd.ac00
Designated bridge has priority 4116, address 000b.befd.ac00
Designated port id is 64.106, designated path cost 0
Timers: message age 15, forward delay 0, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point
Loop guard is enabled by default on the port
BPDU: sent 3, received 379585
----------
Here are port configs.
4506
4506-Root#sh span s
Switch is in rapid-pvst mode
Root bridge for: VLAN0001-VLAN0003, VLAN0006, VLAN0010, VLAN0013-VLAN0018
VLAN0020, VLAN0025, VLAN0032, VLAN0035-VLAN0038, VLAN0075, VLAN0225
VLAN0500
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
EtherChannel misconfig guard is enabled
UplinkFast is disabled
BackboneFast is disabled
Configured Pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 0 0 0 7 7
VLAN0002 0 0 0 7 7
VLAN0003 0 0 0 7 7
VLAN0006 0 0 0 7 7
VLAN0010 0 0 0 7 7
VLAN0013 0 0 0 12 12
VLAN0014 0 0 0 7 7
VLAN0015 0 0 0 7 7
VLAN0016 0 0 0 8 8
VLAN0017 0 0 0 7 7
VLAN0018 0 0 0 7 7
VLAN0020 0 0 0 8 8
VLAN0025 0 0 0 25 25
VLAN0032 0 0 0 7 7
VLAN0035 0 0 0 11 11
VLAN0036 0 0 0 7 7
VLAN0037 0 0 0 7 7
VLAN0038 0 0 0 8 8
VLAN0075 0 0 0 7 7
VLAN0225 0 0 0 7 7
VLAN0500 0 0 0 8 8
---------------------- -------- --------- -------- ---------- ----------
21 vlans 0 0 0 178 178
interface GigabitEthernet2/42
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree vlan 3-500 port-priority 64
end
interface GigabitEthernet2/44
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree vlan 2 port-priority 64
end
3750
interface GigabitEthernet2/0/1
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree vlan 2 port-priority 80
end
interface GigabitEthernet1/0/1
switchport trunk encapsulation dot1q
switchport mode trunk
end
