STP root port tuning on 3750 Stack

[FlorianBerthelot]

Hi Cisco Community,

I have a question regarding Spanning tree tuning. We have a 3750 stack with both te interfaces towards our core (STP root). I need to tune STP so the root port will be te 2/0/1 and not Te1/0/1 (all ports have default config).

I would like to use the STP port priority to do so :

Distri(config)# int te 2/0/1

Distri(config-if)#spanning-tree vlan 2013 port-priority 16

Distri001(config-if)#do sh spanning-tree vlan 2013 interface te 2/0/1

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN2013            Altn BLK 2          16.107  P2p
 

So that looks right, question is why is the port still in ALT BLK ?

Other port has lower priority :

Distri001#show spanning-tree vlan 2013 interface tenGigabitEthernet 1/0/1

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN2013            Root FWD 2         128.53   P2p

I've made the same config on the core but no chance either.

What Am I missing ?

thanks for your help 

Florian

[FlorianBerthelot]

OK according to the guide :

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swstp.html#wp1020470

 If your switch is a member of a switch stack, you must use the spanning-tree [vlan vlan-id] cost cost interface configuration command instead of the spanning-tree [vlanvlan-id] port-priority priority interface configuration command to select an interface to put in the forwarding state. Assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last. For more information, see the "Configuring Path Cost" section.

This should be why it is not working. Question now is :

Anyone know why this does not apply to stacks ?
As these are Te ports, cost is 2, should i lower the cost to 1 the one i want to use or increase to 3 the other one (to have it as alt blk)? Any guidelines or feedback on this ?

Thanks in advance

Florian

[InayathUlla Sharieff]

Anyone know why this does not apply to stacks ?

Answer: The above mentioned in the document is for stack

As these are Te ports, cost is 2, should i lower the cost to 1 the one i want to use or increase to 3 the other one (to have it as alt blk)? Any guidelines or feedback on this ?

Answer: you need to change the cost to 1.( Always in STP lowest is prefered. Which ever interface you want that to be in forwarding state make that port to be lower cost. Else the alternate would be to increase it on the other link ).

HTH