STP Bridge ID

hello. I was wondering if anyone knew how a bridge's MAC address is chosen when determing it's STP bridge ID.

I know it's bridge priority + MAC address, but which MAC address?

Is it the lowest MAC address on the switch?

Any help would be greatly appreciated.

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Comments

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@duking

-it's a fishy thing , a little more complicated to understand , but easy altough.
-that ID is consider to be chosen bridge prioryty ( let's say that all of them have default priotity , if changed to another valued , the ID will be that automaticaly , base of changed pririty , if not is based on lowest MAC address ).

Just consider 111111111111 and 121111111111 , the first one is smaller , try to make the diference betwne them , who's the smallest one will be elected , it the priority's let you this.

I think this may be the answer.. :P

bmauro

Had to search around for this one

OK - just to cover both priority and MAC addresses during the election process.

For each VLAN, the switch with the highest switch priority (lowest numeriacl priority value) is elected as the root switch. If all switches are configured with the defualt priority (3276

, the switch with the lowest MAC address in the VLAN becomes the root switch.

So you basically had it right - it's the lowest MAC address in each VLAN on the switch. Just remember the VLAN part because that could greatly influence if a switch becomes root or not.