Bridge priorities for STP

Answered Question
Apr 16th, 2007
User Badges:

Hi,

I thought priority for building Bridge ID within STP defaults to 32,768.


I have created 4 vlans, and the default priority for vlan 2 and 3 for example is: 32770 and 32771. Why is not the default one? The switch is with its default config.


Thanks in advance.


Jorge

Correct Answer by Jon Marshall about 10 years 2 weeks ago

Hi Jorge


This is probably because you have spanning-tree system extend-id turned on.


Taken from one of our switches :-


HQ-FTZ-F00-SW1#sh spanning-tree vlan 30


VLAN0030

Spanning tree enabled protocol ieee

Root ID Priority 32798

Address 000a.b8b3.a980

Cost 19

Port 24 (FastEthernet0/24)

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


Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)

Address 000e.84e1.f680

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

Aging Time 300


Interface Role Sts Cost Prio.Nbr Type

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

Fa0/16 Desg FWD 19 128.16 Edge P2p

Fa0/17 Desg FWD 19 128.17 Edge P2p

Fa0/24 Root FWD 19 128.24 P2p



This switch has "spanning-tree system extend-id" turned on. This output is for vlan 30 and the priority is


32768 + 30 = 32798.


So in your case


32768 + 2 = 32770

32768 + 3 = 32771


HTH


Jon

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Correct Answer
Jon Marshall Mon, 04/16/2007 - 01:57
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Hi Jorge


This is probably because you have spanning-tree system extend-id turned on.


Taken from one of our switches :-


HQ-FTZ-F00-SW1#sh spanning-tree vlan 30


VLAN0030

Spanning tree enabled protocol ieee

Root ID Priority 32798

Address 000a.b8b3.a980

Cost 19

Port 24 (FastEthernet0/24)

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


Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)

Address 000e.84e1.f680

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

Aging Time 300


Interface Role Sts Cost Prio.Nbr Type

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

Fa0/16 Desg FWD 19 128.16 Edge P2p

Fa0/17 Desg FWD 19 128.17 Edge P2p

Fa0/24 Root FWD 19 128.24 P2p



This switch has "spanning-tree system extend-id" turned on. This output is for vlan 30 and the priority is


32768 + 30 = 32798.


So in your case


32768 + 2 = 32770

32768 + 3 = 32771


HTH


Jon

jorolas Mon, 04/16/2007 - 02:15
User Badges:

Thanks a lot Jon, I had it turned on, but as it is the defautl config, I didnt realize about it.


didnt know about it.


Best regards,


Jorge

Actions

This Discussion