spanning-tree vlan root primary

Unanswered Question
Sep 14th, 2009

in Chris Bryant's BCMSN book, it says:

- if the current root bridge priority is greater than 24576, then our new root bridge priority is 24576

-if the current root bridge priority is less than 24576, then our new root bridge priority is: priority(currentRoot)minus 4096.

However, on my GNS3 lab, I got a priority of 8192, same as stated in cisco 6500 IOS command reference.

So does it depend on the plateform or is there a book mistake?

I have this problem too.
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Giuseppe Larosa Tue, 09/15/2009 - 02:14

Hello Wassim,

if system-id extension is enabled the priority is:

priority vlan x = base value + X

so in this case the base priority is reduced accordingly to 8192


I'm not sure but the book may refer to old way (without system-id extension)

Hope to help


Wassim Aouadi Tue, 09/15/2009 - 07:44

Hi Giuseppe,

Allow me to quote you:

"priority vlan x = base value + X

so in this case the base priority is reduced accordingly to 8192

2*4096 "

I don't really get this one.

the old bridge priority was the default: 32768.

I then issued a "spanning-tree root primary", and it gave me 8192.

I posted the configuration of the Root Bridge and a "show spannin vlan 1 brief".

I'm using a router IOS with NM-16ESW to emulate the switch.

Giuseppe Larosa Tue, 09/15/2009 - 07:58

Hello Wassim,

I see you are using an etherswitch module inserted in a router.

It looks like it is not using the spanning-tree system-id extension

so priority is set to 8192 and the use of the macro results in the following line:

spanning-tree vlan 1 priority 8192

And I have to agree with you that the behaviour is different then what stated in the book.

However, the concept that stays behind "spanning-tree root primary" is that this command is actually a macro and it is executed only one time.

if after this somebody configure another device with a lower priority this node cannot react to this and it will lose its root bridge role.

if you use

spanning-tree vlan 1 priority 0

only another device with pri 0 and a lower MAC address can take the role of root bridge.

For example in most of our campuses we manually set to 0 the priorities for vlans


spanning-tree vlan 1,10,13,20,30,50,101,103,105 priority 1

spanning-tree vlan 11,21,51-52,69,102,104 priority 0


spanning-tree vlan 1,10,13,20,30,50,101,103,105 priority 0

spanning-tree vlan 11,21,51-52,69,102,104 priority 1

so core2 is root bridge for vlan subset (11,21,51-52,69,102,104) and core1 for (1,10,13,20,30,50,101,103,105)

Hope to help



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