spanning tree priority question, please help.

[dannan lin]

i have three switches and one of them was set to root bridge. the settings of the other two switches are like below.

SwitchA(config)#do show spanning-tree vlan 1

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    4097

             Address     0040.0B7C.E1A6

             Cost        19

             Port        1(FastEthernet0/1)

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     0050.0FAE.625D

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

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

Fa0/1            Root FWD 19        128.1    P2p

Fa0/2            Altn BLK 19        128.2    P2p

SwitchB(config)#do show spanning-tree vlan 1

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    4097

             Address     0040.0B7C.E1A6

             Cost        19

             Port        13(FastEthernet0/13)

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)

             Address     0001.C760.D1C7

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

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

Fa0/13           Root FWD 19        128.13   P2p

Fa0/15           Desg FWD 19        128.15   P2p

switch A' s f0/2 is connected to switch b's f0/15 . from above summary, f0/2 is in block status ,and f0/15 is in forward status. i wanted to change that and make f0/2 to forward status and f0/15 to block status.

i tried " spanning-tree vlan 1 root secondary " on switch B but nothing happened?

why is that can any one help? or is there any other command does the trick.

thanks

[Marwan ALshawi · Accepted solution]

Hi

Actually f0/15 is not forwarding while it works like backup

Anyway what you cam do is change the cost of Vlan 1 to something less than 19 on switch b f0/2

spanning-tree Vlan 1 cost 15

HTH

Sent from Cisco Technical Support iPhone App

[cadet alain · Accepted solution]

Hi,

f0/15 is the Designated Port on that segment and it is forwarding.

spanning-tree vlan 1 root secondary will make bridge priority better than switchA but less than the actual root bridge so when the root bridge fails it will take over its role.if you want f0/15 to be blocking so non designated port you must make this switch the designated bridge on the segment.How? make it advertise the least path cost to the root bridge on this segment, so you have to manipulate the cost of the other bridge interface to be higher like Marwanshawi explained above.

Regards.

Alain.

[dannan lin]

thank you for your help.

[dannan lin]

thank you for your help.