Which port will the spanning-tree algorithm select as a bridge’s root port?
- The first port on the root bridge to receive an STP packet
- The port through which the root bridge can be reached with the lowest-cost path
- The port through which the root bridge can be reached with the lowest-value interface identifier
- The port through which the root bridge can be reached with the highest-value interface identifier
Root ports are ports that are in the forwarding state and provide connectivity to the root bridge. The port through which the root bridge can be reached with the lowest-cost path is the root port. All the ports on the root bridge (the bridge with the lowest bridge ID) are in the forwarding state and are referred to as designated ports.
Bridges and switches use the Spanning-Tree Protocol (STP) to prevent network loops. Without a loop-avoidance service on the network, Layer 2 devices, in certain situations, will endlessly flood broadcasts. An STP-enabled device recognizes a loop in the topology and blocks one or more redundant paths, preventing the loop. STP allows the switches to continually explore the network so that the loss or addition of a switch or bridge is also quickly discovered. STP is enabled by default on Catalyst switches.
For example, if two switches have an active connection between them that is forwarding traffic and a second link is connected between the same two switches, one of the two switch ports will go into a blocking state when BPDUs are received on the link. This helps to ensure that a loop does not form using the redundant connections. In some situations, heavy traffic may prevent the reception of BPDUs when the second link is put in place, and in that case, a loop may still form.
The root port is not selected based on the first port to receive an STP packet on the root bridge. Neither is it based on the lowest or highest interface identifier values.
Note: In some situations, there may be two ports with equal cost to the root bridge. When this occurs, the port with the lowest port number becomes the root port.
Layer 2 Technologies
Configure and verify spanning tree