What is the cost for an interface operating at 100 Mbps in an 802.1D STP configuration?

In an 802.1D Spanning Tree Protocol (STP) configuration, the cost assigned to a port is based on the bandwidth of the network link it represents. The lower the cost value, the more preferred that path is in terms of STP calculations.

For Ethernet, the cost values are defined by a standard established by the IEEE. For an interface operating at 100 Mbps, the assigned cost is indeed 19. This value reflects the protocol's method of calculating the total cost associated with traversing a network path based on the bandwidth of the links.

As the data rates increase, the cost values decrease, which influences the STP algorithm's decision-making process when determining the root port and the designated ports for forwarding traffic. Higher bandwidth interfaces naturally have lower costs, thus making them more favorable for active data transmission in the topology.

Understanding these cost assignments is crucial for network design and troubleshooting in environments that utilize STP to prevent loops and ensure efficient data routing.