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Basic Network Fundamental - Change bridge priority without change VLAN ID, Root Bridge, Root port election, Root cost measurement

1) How to change the bridge priority without change the VLAN ID?



  • If you want to change the switch’s bridge priority you can only change the total bridge priority (bridge priority + extended system ID) in units of 4096.
  • The STP bridge priority can only be changed in units of 4096.
  • The valid values you can configure are: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28762, 32768, 36864, 40960, 45056, 49152,
    53248, 57344 or 61440.
  • The extended system ID will then be added to this number to make the total bridge priority value.


2) Why 32768 default bridge priority?


  • It’s because this field is 16 bits in length, and the most significant bit is set 1 by default. Therefore, the default bridge priority was 32768
  • With the addition of the extended-system ID, adding the VLAN ID number to the bridge priority, that changed.
  • So, the default VLAN ID is 1, therefore the bridge priority in total isn’t 32768, its 32769.
  • In the default VLAN of 1, the default bridge priority is 32769, which is 32768+1.


3) What is the root bridge?



  • When a switch is powered on, it assumes it is the root bridge.
  • It will only give up its position if it receives a “superior” BPDU means lower bridge ID.
  • Once the topology has converged and all switches agree on the root bridge, only the root bridge sends BPDUs.
  • The reason all switches send BPDUs at first is because they all think they are the root bridge.
  • Other switches in the network will forward BPDUs from the root bridge, but they will not generate their own original BPDUs.


4) What are the steps are there for select the root port?




Step1:

  • The switch with the lowest bridge ID is elected as the root bridge.
  • All ports on the root bridge are designated ports, so they are in a forwarding state.
  • It’s important that this is the first step that spanning tree takes, because the rest of the steps depend on knowing which switch is the root bridge.


Step 2:

  • Each remaining switch will select ONE of its interfaces to be its root port.
  • So, that means there is one root port on each switch in the network, except on the root bridge.
  • The interface with the lowest root cost will be the root port.
  • Root ports are also in a forwarding state.

5) What is the root cost measurement?


  • The root cost is the total cost of the outgoing interfaces along the path to the root bridge.
  • Each interface has an associated spanning tree ‘cost’.
  • A regular Ethernet interface, with a speed of 10 megabits per second, has a cost of 100.
  • Fastethernet, 100 megabits per seconds, has a cost of 19.
  • 1 Gigabit ethernet has a cost of 4, and 10 gigabit ethernet has a cost of 2.


Below Cost details for understanding


10 Mbps Speed Cost for STP is 100

100Mbps Speed Cost for STP is 19

1Gbps Speed Cost for STP is 4

10Gbps Speed Cost for STP is 2

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