2960 STP mode blocking for no apparent reason

[eschroedorica]

I've run into an odd problem - I have connected two 2960s together with copper on FastEthernet interfaces, and STP on the new switch immediately puts that port into blocking mode. I don't understand why this would be, since there is only one connection between the two, in fact, there is only one connection at all on the switch that is blocking. Does anyone have an explenation for this? I didn't think STP would block unless there is a loop, and there can't be a loop when there's only one link between the devices.

[glen.grant]

  If you have 2 switches and that is the only cable attached to either switch then no it should not be blocking . Are there any other cables attched to either switch ?

[eschroedorica]

Here's what I've got: "Switch A" is the existing device that is in operation. "Switch B" is the one hanging off Switch A.

Switch B has only one cable connected on Fa0/1, all others are empty and down. Switch B Fa0/1 is connected to Fa0/10 on Switch A. Switch B immediately puts that port into blocking mode.

Switch B "show span"

Switch#sh span
VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32778
             Address     0019.2f94.d300
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     0019.2f94.d300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg BKN*19        128.1    P2p Edge *TYPE_Inc 

Switch B Fa0/1 running config:

Switch#sh run int fa0/1
Building configuration...
Current configuration : 84 bytes
!
interface FastEthernet0/1
 switchport access vlan 10
 spanning-tree portfast
end

Switch A is in forwarding mode. Switch A "show span":

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32768
             Address     0015.c626.af00
             Cost        4
             Port        25 (GigabitEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     0027.0ca3.3e00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/3               Desg FWD 19        128.3    P2p Edge 
Fa0/10              Desg FWD 19        128.10   P2p Edge 
Fa0/12              Desg FWD 19        128.12   P2p Edge 
Gi0/1               Root FWD 4         128.25   P2p 
Gi0/2               Desg FWD 4         128.26   P2p Edge 

Switch A Show run int fa0/10:

USRDNZ07#sh run int fa0/10
Building configuration...
Current configuration : 237 bytes
!
interface FastEthernet0/10
 switchport access vlan 10
 switchport mode access
 switchport voice vlan 100
 srr-queue bandwidth share 10 10 60 20
 priority-queue out 
 mls qos trust cos
 auto qos voip trust 
 spanning-tree portfast
end

Any ideas?

[JohnTylerPearce]

What do you see when you run the command show interest status? Also as the previous poster said, if you only have one cable between two switches, one switch will be elected the root switch, and will have nothing but designated ports. The port on the other end, should be A root port. Can you post A show run of each interface and a show span?


Sent from Cisco Technical Support Android App

[eschroedorica]

I've figured it out. I noticed that console output from Switch B was citing port-type inconsistency for reason for the block.

02:37:44: %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk FastEthernet0/1 VLAN10.
02:37:44: %SPANTREE-7-BLOCK_PORT_TYPE: Blocking FastEthernet0/1 on VLAN0010. Inconsistent port type.
Switch#sh spanning-tree inconsistentports 
Name                 Interface                Inconsistency
-------------------- ------------------------ ------------------
VLAN0010             FastEthernet0/1          Port Type Inconsistent
Number of inconsistent ports (segments) in the system : 1

I think since I had a voice vlan configured on the Switch A port, Switch B interpereted it as a trunk port, even thought it was configured as access.

I removed the "sw voice vlan" statement from thh Switch A port, and after a shut/no shut on the interface, it came up without a problem.

Switch#show span
VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32768
             Address     0015.c626.af00
             Cost        23
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     0019.2f94.d300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Root FWD 19        128.1    P2p 

Lesson learned.

[JohnTylerPearce]

For starters links connecting switches, which are considered by story to be point to point links should not be configured for port fast.

Second SWB has been elected the root switch, And on SWA, the best path cost to reach the root is from Gi0/1.

Take out the portfast command on the two Fa ports connecting the two switches and let us know what happens.


Sent from Cisco Technical Support Android App

[JohnTylerPearce]

Depending on the DTP modes, it was probably trying to form a trunk


Sent from Cisco Technical Support Android App