Priority Reducing STP Election (PRSTP) Configuration

STP Configuration in Packet Tracer

RSTP Configuration in Packet Tracer

Priority Reducing STP Election

When both switches are from different or same models then the root bridge selection is depended on priority numbers and then MAC address.If both switches have the same priority numbers then the decisions will go on the MAC address so in this case the Switch AAA is the oldest model and it will become the Root Bridge because of lower priority numbers as well as lower MAC address. The second switch BBB is the latest model and it will become Non-Root Bridge, but it has Gigabit Ethernet ports as compared to the AAA switch which will be the best and the performance will also be good due to its Gigabit Ethernet ports.

Root Bridge Selection

So in this case, we will reduce forcefully the bridge priority to make the Root Bridge, we can’t change the MAC address so we will reduce the priority numbers to make the latest switch as a Root Bridge. Normally, the priority number is 32768 and adding by default VLAN number 1, the priority number will become 32768.

Priority Reducing STP Election

And the command used to reduce the priority number and that command will be applied in the third mode Config Mode. Use the below command on the latest switch to make it the Root Bridge.

Switch (config) # spanning-tree vlan 1 priority 4096

The above command will reduce the priority number of the latest switch and it will become the Root Bridge because the priority number 4096 is lower than 32768.

The range of priority numbers is from 0 to 61440, and why we set 4096 instead of 0 because it is the mathematical concept of the 16 increments of 4096 which start from 0 and it will become the 61440 number when the 16 increment ends.

VLAN Priority Number Range

Add 0 into 4096, it will become 0+4096=4096, and then add 4096 with the previous result which is 4096 it will become the 8192. Add the 4096 with the previous last result 8192 it will become 12288 and do it at 0-15 increments, the total is 16 increments and the last increment will give the result 61440. But we will only do the increment before the 32768 number, we can increment further but the priority value of the latest switch will increase, and then there is no benefit for this activity because we are trying to make the low priority number so that the latest switch can become the Root Bridge and the performance should be best as compare to the old switch.

If we don’t know the priority number but we want to reduce the priority number of the latest switch so that it can become the Root Bridge. For that, we will write the root primary instead of priority 4096 in the Config Mode.

Switch (config) # spanning-tree vlan 1 root primary

Adding the above command, all the switches in the network will be negotiating each other and the switch where we configure the above command will decrease its priority number from other switches, and it will become the Root Bridge.

Basic Configuration of Switches

Switch 1

Switch>enable

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch (config) #hostname SW1

SW1 (config) #end

SW1# show spanning-tree