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New Member

Spanning-tree root bridge

Hi netpro.

i got 1 question which make me very blur now.

i have configure the spanning-tree vlan priority to 8192 at core-switch. but when i show spanning-tree root .. i still see the priority is 8193 .

Vlan Root ID Cost Time Age Dly Root Port

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

VLAN0001 8193 0015.f9be.e400 4 2 20 15 Gi0/1

VLAN0059 8251 0015.f9be.e400 4 2 20 15 Gi0/1

VLAN0200 8392 0015.f9be.e400 4 2 20 15 Gi0/1

anyone who has idea why ?


Re: Spanning-tree root bridge

HI, [Pls rate if helps]

The default priority for switches is 32768. When you set the priority with the command, you force the selection of Switch as the root switch because Core Switch has the lowest priority.

Pls follows as below:

Core-Switch> (enable)set spantree root 1

Core-Switch> (enable)set spantree root 59

Core-Switch> (enable)set spantree root 200

The set spantree priority command provides a another method to specify the root switch:

Switch-15> (enable)set spantree priority 8192 1

If you are not certain that all the switches in your network have a priority that is greater than 8192, set the priority of your desired root bridge to 1.

Hope I am Informative.


Best Regards,

Guru Prasad R

Hall of Fame Super Blue

Re: Spanning-tree root bridge

Hi Jack

This is because you have spanning-tree system extend-id turned on. So this adds the vlan number to the base priority - see below for interpretation of your output.

Taken from one of our switches :-

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


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

vlan 1 = 8192 + 1 = 8193

vlan 59 = 8192 + 59 = 8251

vlan 200 = 8192 + 200 = 8392



New Member

Re: Spanning-tree root bridge

Hi Jon,

thanks for your information , woww.. very helpful and now i understand already.

but, is that any impact to the network ?

your reply will be highly appreciated.



Hall of Fame Super Blue

Re: Spanning-tree root bridge


No real impact but you do have to be careful that all your switches are running sys-id extension so they all make the same priority calculations per vlan.


Re: Spanning-tree root bridge


AFAIK A Catalyst switch can be configured to use one of the following formats for its STP Bridge ID:

1) Traditional 802.1D bridge priority value (16 bits), followed by the unique switch MAC address for the VLAN.

2) The 802.1t extended system ID (4-bit priority multiplier, plus a 12-bit VLAN ID), followed by a nonunique switch MAC address for the VLAN.

If the switch can't support 1,024 unique MAC addresses for its own use, the extended system ID is always enabled by default. Otherwise, the traditional method is enabled by default.

In the extended system-id mode, the default priority is 32,768 plus the VLAN number.And the pririty should be multiple of 4096.



shri :)

New Member

Re: Spanning-tree root bridge

Hi Jon and Shri,

so from your msg, i just have to make sure all switches have enable the spanning-tree extend-id only , right ? so far that's no any impaction to the network , right ?



New Member

Re: Spanning-tree root bridge

So am i right in thinking that for a standard STP setup all that is required is...

Designated root bridge;

spanning-tree mode pvst

spanning-tree vlan xx priority 4096

And all other (non-root) bridges;

spanning-tree mode pvst

spanning-tree extend system-id

I know other things like UDLD and BPDU Guard are useful, but for a basic mesh setup are the above lines enough to be going on with and avoid Loops?

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