Spanning tree default priorities

[tedauction]

Hello, I have a WS-C3650-24PS running rapid-pvst. This has a default priority of 4096.

I also have a WS-C2960-24PC-L  running pvst which has a default priority of 32768.

Why are there two differnet default spanning-tree priorities ? Does rapid-pvst use 4096 instead of 32768 as a default priority ?

Thank you.

[Peter Paluch · Accepted solution]

Hello,

To my best knowledge, regardless of the STP version (legacy STP, RSTP, MSTP, Per-VLAN STP, Per-VLAN RSTP), the default priority is 32768. I am quite surprised to hear that the Cat3650 uses the value of 4096 as its default priority.

Would it be possible for you to post the following two outputs from the Cat3650?

show run | i span
show spanning-tree bridge

I would like to double-check if the the priority is truly unchanged, and at what value it is currently.

Best regards,
Peter

[tedauction]

yes, sorry, indeed someone had manually entered the priority 4096.

[Vasanthakumar]

Hi Peter,

I have question that why they select 32768 as default bridge priority, what is the reason behind this???

Thank you

[Julio E. Moisa]

Hi

The 32768 value is obtained using  the Bridge priority included into itself, explanation below:

Bridge priority (2 Bytes = 16 bits)

Bridge priority is created with Bridge Priority (4 bits) and Extended System ID (12 bits).

If we convert it to binary we will have: 

B = Bridge priority

E = Extended System

BBBB.EEEEEEEEEEEE

E = 2^12 = 4096

We can combine the "B" bits for example:

0001 = 1

0010 = 2

0011 = 3

0100 = 4

0101 = 5

0110 = 6

0111 = 7

...

1111 = 15

These values will be multiplied with the extend system ID value (4096)

1 * 4096 = 4096

2 * 4096 = 8192

3 * 4096 = 12,288

4 * 4096 = 16,384

....

7 * 4096 = 28672

8 * 4096 = 32768  default value selected. 

...

15 * 4096 = 61440 máximum value

As you can note it is multiple of 4096. Also you can have 0 =  0 * 4096, where 0 is a valid value of STP priority on the switches. 

Hope it is useful

:-)

[Vasanthakumar]

Thank you so much for replying and clarifying my query

[Julio E. Moisa · Accepted solution]

Hi

It looks like the priority and STP mode has been modified on the Cisco 3560 by someone. 

The default STP mode on the most of Cisco Catalyst switches is pvst, and rapid-pvst on Nexus. 

The default priority is 32768

I don't have knowledge about your infrastructure, but if the 3560 is the root bridge for the VLANs I suggest change the STP mode on the 2960 to rapid-pvst as well. 

Hope it is useful

Happy new year.

:-)