The diagram below shows the state of the switch interfaces after STP has converged.
Based on the interface states, which of the following statements are true? (Choose all that apply.)
A. The Fa0/2 interface on SW 2 is a designated port
B. SW 3 is the root bridge
C. SW 2 is the root bridge
D. The Fa0/0 interface on SW 2 is a designated port
E. The Fa0/0 interface on SW 2 is a root port
Correct Answer: BD
Convergence has occurred in a spanning-tree network when all switch ports are in either a forwarding state or a blocking state (known as discarding state in RSTP). You can use the location of these blocked and forwarding ports to infer the location of the root bridge and the state of any unlabeled ports in the diagram.
SW3 is the root bridge and the Fa0/0 interface on SW2 is a designated port. It can be determined that SW3 is the root bridge because all of its ports are in a forwarding state. Any switch that has at least one port blocking (such as SW1 and SW2) are non-root bridges. As there must be a root bridge, that leaves SW3 as the only candidate.
After establishing that SW3 is the root, it can be determined that the connection between SW1 and SW2 is a segment that does not have a direct connection to the root bridge. These sections must have one end set as a designated port and thus set to forward. Since the Fa0/0 interface on SW2 is forwarding, it is the designated port for that segment.
The Fa0/2 interface on SW2 is not a designated port. The interface on each non-root switch with the lowest cost path to the root bridge will be the root port. Since SW3 is the root bridge, the connection to SW3 via Fa0/2 is the lowest cost path to the root bridge for SW1 and thus is a root port, not a designated port. Moreover, designated ports only exist on segments that do not have a direct connection to the root bridge.
SW2 is not the root bridge. One of its ports is blocking, which will not occur on a root bridge.
The Fa0/0 interface on SW2 is a not root port. It is the designated port for the segment between SW1 and SW2.
The process of determining these port states occurs in this order:
1. Selection of the root bridge. When all bridge priorities have been left to their default, all switches will have same bridge priority. When that is the case, the switch with the lowest MAC address will be selected root bridge. ALL ports are in a forwarding state on the root bridge, which explains why all of the ports on SW3 will be in a forwarding state.
2. Determination of the root ports on each non-root bridge. Each non-root bridge will select the interface it possesses with the least cost path to the root bridge. Once selected, that port will be placed in a forwarding state.
3. Determination of the designated port on each segment that does not connect directly to the root bridge. There is one such segment in the diagram (SW1 to SW2). The interface on either end of the segment that has the least cost path to the root bridge will be the designated port for that section. It may have several paths, but the least cost path is used in the determination of the designated port for the segment.
Once determined, the designated ports will be set to forwarding, and all ports that are neither root nor designated ports will be set to blocking.
LAN Switching Fundamentals
Configure, verify, and troubleshoot STP protocols
Cisco > Internetworking Technology Handbook > Bridging and Switching > Transparent Bridging > Spanning-Tree Algorithm