How port priority is influencing root port selection?

[Wajih Rehman]

Hello,

I have created 4 trunk interfaces between switches. What I did is to change the priority of one port to 64 and expected that it will become the root port but it did not happened.

Then I shutdown the root port Et1/0   and Et1/1 came as a root port. I was all the time thinking that it is the port priotity which will force Et1/3 to become root.

Can anyone explain? Thanks for your support in advance.

VLAN0011
  Spanning tree enabled protocol ieee
  Root ID    Priority    24587
             Address     aabb.cc00.0200
             Cost        100
             Port        5 (Ethernet1/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32779  (priority 32768 sys-id-ext 11)
             Address     aabb.cc00.0300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et1/0               Root FWD 100       128.5    P2p 
Et1/1               Altn BLK 100       128.6    P2p 
Et1/2               Altn BLK 100       128.7    P2p 
Et1/3               Altn BLK 100        64.8    P2p 
Et2/0               Altn BLK 100       128.9    P2p 
Et2/1               Altn BLK 100       128.10   P2p 
Et2/2               Altn BLK 100       128.11   P2p 
          
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------

Et2/3               Altn BLK 100       128.12   P2p 
Et3/1               Desg FWD 100       128.14   Shr Edge 

[Rolf Fischer · Accepted solution]

Hello,

welcome to the CSC forum!

I was all the time thinking that it is the port priotity which will force Et1/3 to
become root.

Well, that's true. In case of equal path costs to the root and the same sending Bridge ID, the port priority is the last tie-breaker in the root port election process. But it is not the port priority of the local port but the sending bridge's port priority. If you want to change the root port by a local parameter, you have to use the port cost.

HTH

Rolf

[Wajih Rehman]

Thanks so much

[Rolf Fischer]

You're welcome. And thanks for using the rating system!

[EDIT] When re-reading my posting I found a little mistake: I should have said "the port ID is the last tie-breaker", not the port priority.

The priority is just the configurable part of the port ID, the very last tie-breaker is the fixed part - the portnumber.