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With STP one of the port of the access switch connecting to the backup path should be blocked

Dear All,

 

I have setup my LAN with one L3 cisco 3750 and have 2 2960 as access switch. The two access switch are connected to the L3 switch directly  and two access switch connected back to back for redundancy. There one VLAN configured between all these switches.

When I checked the show spanning-tree one of the access switch the ports is correct as below,

sw1#sh spa
VLAN0005
  Spanning tree enabled protocol ieee
  Root ID    Priority    24581
             Address     0001.4353.DB5A
             Cost        4
             Port        25(GigabitEthernet1/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32773  (priority 32768 sys-id-ext 5)
             Address     0001.C760.93AC
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1            Root FWD 4         128.25   P2p
Gi1/2            Altn BLK 4         144.26   P2p

 

whereas the in the other L2 switch both the port are in forwarding state. 

sw2#sh spanning-tree 
VLAN0005
  Spanning tree enabled protocol ieee
  Root ID    Priority    24581
             Address     0001.4353.DB5A
             Cost        4
             Port        25(GigabitEthernet1/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32773  (priority 32768 sys-id-ext 5)
             Address     0001.4256.9A77
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1            Root FWD 4         128.25   P2p
Gi1/2            Desg FWD 4         144.26   P2p

 

I want the redundant port to be blocked state. I tried changing the port priority of the access switch but did not yield any result.

Request for support.

Easwar

 

 

 

2 REPLIES

Hey Easwar,Provide show cdp

Hey Easwar,

Provide show cdp nei from all three switches.

Regards,

RS.

Cisco Employee

Hi Easwar,What you see is STP

Hi Easwar,

What you see is STP working properly, and the port roles and states are exactly as they should be. Even on a redundant link between switches, exactly one port must be a Designated port in the Forwarding state. STP never blocks a redundant link between switches on both ends. First, it is not necessary to prevent loops: as long as your Sw1 has its port in the Blocking state, it will not be using that link to forward data. Second, if there was an unmanaged switch connected between Sw1 and Sw2 with some stations attached, and both Sw1 and Sw2 had their ports in the Blocking state, the stations connected to this unmanaged switch would be entirely cut off from the network.

Is there any particular reason why you want the port to be blocking?

Best regards,
Peter

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