What is the cost associated with a 1 Gbps interface in an 802.1D STP network?

In an 802.1D Spanning Tree Protocol (STP) network, the cost associated with a link is determined based on the speed of the link. This cost is used by STP to choose the best path to forward data and to prevent loops in the network.

For a 1 Gbps interface, the designated cost is indeed set at 4. This cost value is part of Cisco's implementation of the IEEE 802.1D standard, which outlines how to calculate the cost of a path based on the interface speed. The cost values reflect the inverse relationship between bandwidth and cost: as the bandwidth increases, the cost decreases, making higher-speed interfaces more favorable in path selection.

The other options correspond to different interface speeds, with higher costs associated with lower bandwidth interfaces. For example, a 100 Mbps interface typically has a cost of 19, while interfaces with significantly higher speeds such as 10 Gbps might have an even lower cost. Thus, for a 1 Gbps interface, a cost of 4 is appropriate, which aligns perfectly with the principles of STP.