Spanning tree protocol question 1

[andyluk]

Hi,

I'm currently studying for the ICND test, i have a question regarding STP. I understand how STP works, but can someone make it clear in my mind where the bridge's priority value comes from, that combined with the MAC address makes the bridge ID. I think really my questions from this are.
1. What makes the default value 32768?
2. Is the bridge's priority value something that should be manually set when setting up the segment?
3. What is the priority value range?


thanks,

Andy.

 

[Jsteve]

The BID is made up of two parts
-the priority
-the sending bridge's mac address

1. What makes the default value 32768?

The priority number is a 16 bit field so the max number you get for a priority is (2^16 =65536), if you cut that number in half you get your defaukt priority number 32768. which is = to 15 bits (2^15)

2. Is the bridge's priority value something that should be manually set when setting up the segment?

Depending on your topology design, you only want to adjust (lower) the priority of your best switch so that it wins the root bridge election. Also you may opt to raise the priority of your weakest switch so that it will nevr win an election. All others should remain as is.


3. What is the priority value range?

See answer to #1

the range is 0 - 65536 (note* if you set a priority to 0 it will not participate in the root bridge election. This is like shutting off the priority)

 

[andyluk]

Thanks for your help, that answers my question spot on.


Just to expand on your last point. In what circumstance would you shut off the prioity, is this just to make sure a bridge isn't elected whilst creating less overhead in the election process?


Andy.

 

[Jsteve]

I tend to shut off the priority on swithes that I never want to win the root election. For example: if you have 6 switches connected in a 2 (distribution) to 4 (access) topology and all of your switches are 6500s, except one of the access switches which is a 4000. I would turn off the prority on the 4000. So if/when the current root switch fails or is shut down for IOS upgrades, the 4000 would never have a change of taking over the root bridge functions.

 

[andyluk]

Thanks for the help.

Andy.

 

[TheFinalWord]

I really wouldn't disable any switch's ability to become the root bridge. In doing so, you are assuming that another switch will always be reachable via R/STP.

My experience has lead me to set core switches to 8192, distribution to 16384, and access to 32768 (should be factory default, so no touch needed, other than to set to correct mode [rapid, PVST, RPVST/PVRST, etc.]).

The word "always" tends to be one's undoing.