Blocking Port

[amh4y0001]

Device and OS Info :c2960x-universalk9-mz.152-2.E6.bin

On this switch I have a port which is always in amber color and never get green. I have checked and changed the cables but that didn't helped. Interface if gigabit and running on full duplex.

However, I just checked the STP status and found the following (blocking), .. can someone advise what may be possible cause for this?

Switch#sh spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address 00ac.7002.ae00
Cost 8
Port 159 (GigabitEthernet3/0/47)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0000.2b87.4000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi3/0/45 Desg FWD 4 128.157 P2p
Gi3/0/46 Desg FWD 4 128.158 P2p
Gi3/0/47 Root FWD 4 128.159 P2p
Gi3/0/48 Altn BLK 4 128.160 P2p

[luis_cordova]

Hi @amh4y0001 ,

In a nutshell:
Spanning-tree is a protocol that avoids the layer 2 loop.
For this, when detecting a possible layer 2 loop, choose the shortest path, according to the cost of the interfaces to the switch designated as root and block the other paths by putting some ports in block state.

I suggest you check your connections, since the ports in block state are evidence that there is a possible layer 2 loop in your network topology.

Regards

[paul driver]

Hello

---

@luis_cordova wrote:

I suggest you check your connections, since the ports in block state are evidence that there is a possible layer 2 loop in your network topology.

---

STP is doing what's is suppose to do , No evidence at this time to suggest of any L2 loop

[paul driver]

Hello

stp convergence is based on receiving and sending bpdus and in this case that port which is alt-blk is receiving a more desirable bpdu priority then it’s sending and as a result stp has made this port an alternative backup port meaning if for some reason there is failure to the root port this alt-blk port would provide the alternative path back to the stp root switch.

[Leo Laohoo]

Post the complete output to the command "sh interface Gi 3/0/47".

[Deepak Kumar]

Hi,

STP protocol is designed to prevent the Layer 2 loops in the network. Here we are sure that "Gi3/0/48 Altn BLK 4 128.160 P2p" network design is making layer 2 loops. Now, this port is as a backup purpose and if your port between SW1 and SW3 or SW1 and SW2 went down, this port will take care of traffic and become in Forwarding.

 

an example above Picture. Switchport3 Fas0/1 is in the blocking state. I am not sure that your topology is matching with above or not. You may have some other type of topology as well. 

For better understanding your topology, we need some logs from all switches.

sh spanning-tree

Show CDP neighbor details

Regards,

Deepak Kumar

[amh4y0001]

Thanks everyone who contributed in this thread.

Instead of troubleshooting, I have re-desingned the LAN and everything seems to be working fine.