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Community Member

Port blocked by STP

Hello guys,

This is my first time to post here on Cisco forums.

Our company bought a Cisco 2960-S to add to the network, We gave the interface VLAN 1 an IP address, and tried to attached the switch to a jack port on the wall to test if it gets a link or not. The status of the port blinks amber and stays contionous amber, I have searched the manual of the switch and it says that amber means that the port is being blocked by STP.

How can I get this to work.

Thanks and Regards,

Halaakajan

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4 REPLIES
Cisco Employee

Port blocked by STP

Hello,

The amber light can be present because of different reasons. Can you provide the output of the following commands entered on your switch?

Assuming that the port you are connecting is fa0/1, then the commands I am interested in seeing the output are:

show spanning-tree interface fa0/1

show interface fa0/1 switchport

show interface fa0/1 status

Substitute fa0/1 with the name and the number of the real port.

Thank you!

Best regards,

Peter

Community Member

Re: Port blocked by STP

Hello Peter,

This was the output.

Switch#show spanning-tree interface gigabitEthernet 1/0/1

Vlan                Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

VLAN0001            Desg BKN*4         128.1    P2p *TYPE_Inc

Switch#show interfaces gigabitEthernet 1/0/1?

.  :  <1-52>

------------------------------------------------------------------------------------------------------------

Switch#show interfaces gigabitEthernet 1/0/1 switchport

Name: Gi1/0/1

Switchport: Enabled

Administrative Mode: dynamic auto

Operational Mode: static access

Administrative Trunking Encapsulation: dot1q

Operational Trunking Encapsulation: native

Negotiation of Trunking: On

Access Mode VLAN: 1 (default)

Trunking Native Mode VLAN: 1 (default)

Administrative Native VLAN tagging: enabled

Voice VLAN: none

Administrative private-vlan host-association: none

Administrative private-vlan mapping: none

Administrative private-vlan trunk native VLAN: none

Administrative private-vlan trunk Native VLAN tagging: enabled

Administrative private-vlan trunk encapsulation: dot1q

Administrative private-vlan trunk normal VLANs: none

Administrative private-vlan trunk associations: none

Administrative private-vlan trunk mappings: none

Operational private-vlan: none

Trunking VLANs Enabled: ALL

Pruning VLANs Enabled: 2-1001

Capture Mode Disabled

Capture VLANs Allowed: ALL

Protected: false

Unknown unicast blocked: disabled

Unknown multicast blocked: disabled

Appliance trust: none

Switch#show interfaces gigabitEthernet 1/0/1 status

Port      Name               Status       Vlan       Duplex  Speed Type

Gi1/0/1                         connected 1              a-full    a-1000 10/100/1000 BaseTX

Cisco Employee

Re: Port blocked by STP

Hello,

The TYPE_Inc indication tells you that your port is receiving BPDUs from many VLANs (a so-called PVST+ or RPVST+ BPDUs) but the port itself is configured as an access port, not a trunk port. This is the reason the port gets blocked - it is a prevention against potential switching loops caused by the inability of the access port to process PVST+ BPDUs correctly.

Obviously, the wall jack appears to be connected to a trunk port somewhere. If you want to make this link "just work", you need to

  1. configure this port as a trunk port (using the switchport mode trunk command)
  2. create appropriate VLANs on your switch

As I understand this is only a basic connectivity test, it surprisingly tends to start complicate itself. No wall jack should provide a trunk connectivity just so - I wonder how come that the wall jack you're using is a trunk link. That is a very dangerous situation.

Best regards,

Peter

Re: Port blocked by STP

Hi Ramit,

the fact that DTP fails in its negotiation should bring you to check the other end of this link. Can you tell us where you are connecting this switch to? It would be useful to post the same info of the other interface (the one connecting G1/0/1)

Alessio

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