STP's Bridge ID

STP, RSTP, MST, PVST all share one common thing which is almost never discusses and rarely documented. The priority value in the BID.

MST - Multiple spanning-tree.

I had to dig to find stuff on this, there isn't a lot of documentation around for MST. A good video that explains the basics can be found here: http://youtu.be/U2CNahYexlg

Cisco's documentation: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml#diag2

I'm just off to configure this on my rack and see how things go. 

But back the main topic, STP's BID.

Okay so we know the BID of a switch is the combination of the priority value and the mac address. A normal BID usually looks like this:

The zero is the vlan number, aka system-extended-id used in pvst and rapid-pvst

We’ll come back to that shortly.

We also know how this combination works to determine the root STP bridge:

1. Lowest priority wins
2. If priorities are equal then lowest MAC addy wins

But where did this default priority come from? And why does it have to be 32768? A bit weird isn’t it? I didn’t pay much attention to it before, I accepted it for what it was, accepted that it must be for some reason but didn’t care too much. But it really annoyed me when I started tweaking PVST to do load balancing.

In the below figure the following lines were entered on switch A and switch B

Switch-A (config) # spanning-tree vlan 20 root primary

Switch-B (config) # spanning-tree vlan 20 root secondary

Switch-B (config) # spanning-tree vlan 10 root primary

Switch-B (config) # spanning-tree vlan 10 root secondary

Load balancing done for vlan 10 and 20 – beautiful

But as I said before, the priority value is so strange, why the random number, or is it not random?

After some reading I learned back in the old days before PVST the priority value was a flat decimal of 65536, the standard 16 bit number that’s applied to almost everything networking related

PVST uses 12 bits for the system-extended-id leaving 4 bits for the priority value, why?

Well that’s how PVST’s automation works, you see, by stealing 12 bits from the 16 bit value of 65536 we’re able to assign 4096 vlans per priority group… AHHHH so now you see why the maximum vlan range is 4096. Now if you’re wondering what I mean by priority group, I mean the remaining 4 bits.

Confused? Lol

Okay first jump on a switch and enter this:

Switch (config) # spanning-tree vlan 1 priority ?

The output shows you can only go up in increments of 4096

4096

8192

16384

32768

Notice that’s 4 numbers,

Notice the lower number is the usual 4096 seen in vlan configurations

Notice the 32768 is the default priority value for spanning-tree

So it’s not random eh?

This means, as per our diagram, the manual setting of root primary and root secondary respectively which results in 28672 and 24576 must be a calculation of 4096 and yes it is.

They both sit at the power of 7 and the power of 6 respectively

32768 = 8 squared

28673 = 7 squared

24576 = 6 squared

And so on ….

So if you look at it,

The default is 8, setting the root manually drops one increment of 4096, and setting the secondary drops another increment.

Now this explains the randomness, which I guess isn’t random

Side note: if you did your calculations you’d realize the maximum increment is 15 which means you can only set 15 manual spanning-tree bridges in your switch broadcast domain via the priority value.

I suppose you could change MAC addresses, but that’s a bit extreme for STP I would think… what are the chances of 15 bridges failing, and even then if 15 bridges fail I’m sure you wouldn’t give a rats ass about vlan traffic flow, at that point.

Any questions are welcomed

Cheers