CCNA – STP
Note: If you are not sure about Spanning Tree Protocol, please read our Spanning Tree Protocol STP Tutorial (Premium Tutorial).
First by comparing their MAC addresses we learn that switch B will be root bridge as it has lowest MAC. Therefore all of its ports are designated ports -> C & D are correct.
On the link between switch A & switch C there must have one designated port and one non-designated (blocked) port. We can figure out which port is designated port by comparing their MAC address again. A has lower MAC so Fa0/1 of switch A will be designated port while Fa0/1 of switch C will be blocked -> B is correct.
The path cost to the root bridge is the most important value to determine which port will become the root port on each non-root switch. In particular, the port with lowest cost to the root bridge will become root port (on non-root switch).
Per VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. It means a switch can be the root bridge of a VLAN while another switch can be the root bridge of other VLANs in a common topology. For example, Switch 1 can be the root bridge for Voice data while Switch 2 can be the root bridge for Video data. If designed correctly, it can optimize the network traffic.
If we connect two switches via 2 or more links and do not enable STP on these switches then a loop (which creates multiple copies of the same unicast frame) will occur. It is an example of an improperly implemented redundant topology.
Answer A is not correct because Host A & B can access the server in VLAN 9 via Switch1 -> Switch2 -> Switch3 path.
Answer B is not correct because VLAN3 can communicate with other VLANs via port Fa0/7 on Switch1
Answer C is not correct because Host B can transfer files to VLAN 9 via Switch1 -> Switch2 -> Switch3 path. All the links on this path are FastEthernet so we cannot it would be significantly slower.
Answer D is the best choice as Fa0/9 on Switch1 may be the main path to reach VLAN 9 and it is in forwarding state. If the cable connected to Fa0/9 is disconnected, STP needs less than a minute to re-converge (moving from blocking -> listening -> learning -> forwarding state). Then the network function would resume and Host B will access VLAN 9 via Switch1 -> Switch2 -> Switch3 path.
PVST+ is based on IEEE802.1D Spanning Tree Protocol (STP). But PVST+ has only 3 port states (discarding, learning and forwarding) while STP has 5 port states (blocking, listening, learning, forwarding and disabled). So discarding is a new port state in PVST+.
Spanning Tree Protocol convergence (Layer 2 convergence) happens when bridges and switches have transitioned to either the forwarding or blocking state. When layer 2 is converged, root bridge is elected and all port roles (Root, Designated and Non-Designated) in all switches are selected.