Spanning Tree Protocol Questions and Answers

Answer: A

Correct answer: 1, 3 and 5

The three functions of a layer 2 switch are address learning, forwarding and filtering, and loop avoidance. Learning and forwarding are what increase the available bandwidth, by sending each frame only where it needs to go instead of to every port, and loop avoidance through STP is what makes the redundancy safe. Routing and IP addressing are layer 3, and creating loops is the fault being prevented.

Answer: B

Correct answer: STP

STP is the layer 2 loop-avoidance protocol: it elects a root bridge and blocks the ports that would otherwise complete a loop, leaving one active path. VTP distributes VLAN information, RIP is a layer 3 routing protocol, and CDP discovers directly connected Cisco devices.

Answer: D

Correct answer: In an improperly implemented redundant topology

Without STP a redundant topology gives a frame more than one path, so copies circulate and are delivered repeatedly - and because an Ethernet frame carries no time-to-live, nothing stops them. Multiple frame copies, MAC table instability and broadcast storms are the three classic symptoms of a layer 2 loop, and they appear together.

Answer: C

Correct answer: Add the source address and port to the MAC address table and forward the frame out the destination port

The two halves of the switch’s job happen on every frame: it learns from the source address and forwards on the destination. Here the source is new, so the table gains an entry, and the destination is already known, so the frame goes out that one port rather than being flooded. Nothing about the unknown source causes a drop or a flood.

Answer: D

If you have a server or other devices connected into your switch that you're totally sure won't create a switching loop if STP is disabled, you can use something called portfast on these ports. Using it means the port won't spend the usual 50 seconds to come up while STP is converging.

Answer: D

Correct answer: All of the above

All four are genuine properties of layer 2 switching. Forwarding decisions are made in an ASIC rather than in software, so they run at wire speed with latency measured in microseconds, and because the function is simple the silicon is cheap. That combination is why switching displaced software bridging and why switch ports cost a fraction of router ports.

Answer: C

Correct answer: Broadcast storms will occur on the switched network.

Two links between the same pair of switches with STP disabled is a loop. A broadcast is flooded out of both, returns on the other, and is flooded again, multiplying until the links and the switch CPUs saturate. The switches do not load-balance - that requires an EtherChannel, which bundles the links into one logical interface so that no loop exists.

Answer: A

Correct answer: The port is experiencing errors.

An alternating green and amber LED signals errors on the link - collisions, CRC failures or a duplex mismatch. Solid green is a good link, amber alone usually means the port is blocked by spanning tree, and an unlit LED means no link or an administratively shut port. The counters in show interface will say which error is accumulating.

Answer: C

Correct answer: Floods the network with the frame looking for the device

An unknown unicast is flooded out of every port except the one it arrived on, because the switch has no other way of reaching a destination it has not yet learned. The reply then teaches the switch where that device is, and subsequent frames are forwarded rather than flooded. Dropping the frame would break connectivity to any device that has not yet spoken.

Answer: C

Convergence occurs when all ports on bridges and switches have transitioned to either the forwarding or blocking states. No data is forwarded until convergence is complete. Before data can be forwarded again, all devices must be updated.