spanning-tree vlan root primary

[Wassim Aouadi]

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?

[Giuseppe Larosa]

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

2*4096

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

Hope to help

Giuseppe

[Wassim Aouadi]

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]

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

core2:

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

core1:

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

Giuseppe

[tolqabaqci]

Hello there.

When you use the Root Primary command in the Spanning Tree Protocol, the Priority value is automatically set on the Vlan. If you have a small network structure, for example, you have two main switches and you will not have any problems if you configure SW1 as Root Primary and SW2 as Root Secondary.

From the following link, you can browse the relevant article for Root Primary configuration.

Root Primary Configuration