The switch which has the lowest bridge ID can be the root bridge,and the bridge ID is made of priority and MAC address,that is ,
bridge ID = priority + MAC address
So we will choose the switch which has the lowest priority to be the root bridge,if priority of all switches are the same,we will choose the switch which has the lowest MAC address to be the root bridge.
So my question are
1. Why should we choose the switch which has "the lowest" bridge ID to be the root bridge,not "the highest" ?
2.Why can one of switches has the lower priority?if priority can change,then base on what reason,so we let this switch has the lower or lowest priority ? The position in the topology? the performance of switch?
Best Answer
The reason is simply because lower numbers come first when counting forward. For example: 1, 2, 3, 4, etc.
1 came first, therefore it is first in line for priority. 2 came after it, so that’s second in line for priority. It’s just a matter of thinking of the priority order the same way that you increment numbers when counting forward. Whichever came first is first in line.
Your switch priorities should be based on which switch you want to be the root for any particular VLAN ID. For example, if you have systems using VLAN 100, 200, and 300 in your main building but systems using VLAN 400 are in a separate building, you don’t want the systems using VLAN 400 to have to go across a trunk to the main building for those switches to process the traffic and then the traffic come back to the switch the systems are actually connected to, adding a delay, etc., so you set the root for VLAN 400 to be the switch that’s physically where the systems using VLAN 400 are at. The inverse is true for VLANs 100, 200, and 300 - the root for those would be on the main switch and not the switch where VLAN 400 is being used at. That’s just a rough example of why you set the priorities statically.