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Spanning Tree Root Port Election

Spanning Tree Root Port Election: How does it work?

We said that non-tooy ridges have one Root Port. But how is it chosen?

The Root Port is elected by the following criteria:

  1. the port that receives the superior BPDU
  2. the port with the lowest Root Path Cost
  3. the port that received the BPDU containing the lowest sender’s Bridge ID
  4. the port that receives the BPDU with the lowest sender’s port ID

To understand this, here is a simple two-switch setup:

spanning-tree-root-port-election1

Both switches announce themselves as a root bridge. Then switch B receives a superior BPDU on both its interfaces.

  • both ports on switch B receive a same superior BPDU –> tie
spanning-tree-root-port-election2

Superior BPDU received on Fa0/2

spanning-tree-root-port-election3

Superior BPDU received on Fa0/1

  •  since both ports received the same superior BPDU, there is a same Root Path Cost –> tie
spanning-tree-root-port-election4

on Fa0/1

spanning-tree-root-port-election5

on Fa0/2

 

  •  both interfaces connect directly to the root bridge. So they receive BPDU frames with a same Bridge ID –> tie
2015-11-08 07_58_01-spanning-tree-protocol-16-sender-bridge-id

on Fa0/2

spanning-tree-root-port-election6

on Fa0/1

  •  the last chance is to verify which is interface receives a BPDU with the lowest sender’s port ID. Fa0/1 receives a BPDU with Port ID of 32769. Fa0/2 receives a BPDU with Port ID of 32770.
spanning-tree-root-port-election7

on Fa0/1

spanning-tree-root-port-election8

on Fa0/2

The Port ID in the BPDU received by Fa0/1 is the lowest, so Fa0/1 becomes the Root Port (in the Forwarding state) and Fa0/2 is put in the Blocking state. Notice the amber color:

spanning-tree-root-port-election9

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