Spanning Tree Root Bridge

[siksika55]

I am working towards my CCNA and I have a question about root bridge priorities. Several references have conflicting guidance on setting the root bridge priority. Everyone agrees that the default is 32768 and if all are at default, the lowest MAC wins. In the Sybex CCNA book I learned that the root bridge priority had to be in multiples of 4096. Two other reference books do not talk about the 4096 blocks and answers to sample questions ignore the block. Here is one example:
Which of the following Bridge IDs would win election as root, assuming that the switches with the bridge IDs were in the same network?
A. 32768:0200.1111.1111
B. 32768:0200.2222.2222
C. 200:0200.1111.1111
D. 200:0200.2222.2222
E. 40000:0200.1111.1111
F. 40000:0200.2222.2222
I answer A based on default priority and lowest MAC address since C, D, E, and F, all have invalid priorities. The book says C based on priority and the lower MAC than D.

When you lower the priority, can you use any number between 1 and whatever the max size is, or does it have to be in blocks of 4096 ( 0, 4096, 8092, 12288, etc)? I ask this because if you answer the root bridge wrong, other follow-on questions could also be wrong.All help is appreciated.

[lammle]

This is a great question, and one you just have to understand!

Here is an output from a Cisco catalyst switch:
Switch(config)#spanning-tree vlan 1 priority ?
<0-61440> bridge priority in increments of 4096
Switch(config)#spanning-tree vlan 1 priority 200
% Bridge Priority must be in increments of 4096.
% Allowed values are:
0 4096 8192 12288 16384 20480 24576 28672
32768 36864 40960 45056 49152 53248 57344 61440

From the above output, you can see that that book is wrong. See I don't make this stuff up (most of the time :D).

However, please understand that you will see a priority of higher then the listed numbers above. This is because of what is called the system ID extention. Take a look:
Switch(config)#do show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 4121
Address 0012.800c.54c0
Cost 42
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 000d.289f.e100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

You can see that the Bridge ID Priority is 32769 (priority 32768 sys-id-ext 1). The default prioroty plus the vlan, which in this example is 1. For the root bridge, you can see the priority is 4121. The bridge is set to 4096 plus the vlan 25.

I wrote a whole lot about this in my new book, so I recommend taking a look at the switching chapters.

Cheers!
Todd Lammle

[jasos.karamchand]

This is a great question, and one you just have to understand!

Here is an output from a Cisco catalyst switch:
Switch(config)#spanning-tree vlan 1 priority ?
<0-61440> bridge priority in increments of 4096
Switch(config)#spanning-tree vlan 1 priority 200
% Bridge Priority must be in increments of 4096.
% Allowed values are:
0 4096 8192 12288 16384 20480 24576 28672
32768 36864 40960 45056 49152 53248 57344 61440

From the above output, you can see that that book is wrong. See I don't make this stuff up (most of the time :D).

However, please understand that you will see a priority of higher then the listed numbers above. This is because of what is called the system ID extention. Take a look:
Switch(config)#do show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 4121
Address 0012.800c.54c0
Cost 42
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 000d.289f.e100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

You can see that the Bridge ID Priority is 32769 (priority 32768 sys-id-ext 1). The default prioroty plus the vlan, which in this example is 1. For the root bridge, you can see the priority is 4121. The bridge is set to 4096 plus the vlan 25.

I wrote a whole lot about this in my new book, so I recommend taking a look at the switching chapters.

Cheers!
Todd Lammle

Thanks a lot Mr. Lammle
Never knew that.....

[muddyfox1009]

This is a great question, and one you just have to understand!
...

Root ID Priority 4121
Address 0012.800c.54c0
Cost 42
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
..
..
For the root bridge, you can see the priority is 4121. The bridge is set to 4096 plus the the vlan
...
Todd Lammle

Hi, where did that number 4121 comes from ? (is it 4096 + 25?) So why 25? and where did it come from plz.

Aha!! if the Vlan is 26, then the number will be 4122!! This must be correct (?)