Spanning Tree Protocol Questions and Answers

Q: Layer 2 switching provides which of the following? 1.Hardware-based bridging (ASIC) 2.Wire speed 3.Low latency 4.Low cost

Correct answer: All of the aboveAll 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.

Q: Your switch has a port status LED that is alternating between green and amber. What could this indicate?

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.

What is the purpose of Spanning Tree Protocol in a switched LAN?

A.To provide a mechanism for network monitoring in switched environments
B.To prevent routing loops in networks with redundant paths
C.To prevent switching loops in networks with redundant switched paths
D.To manage the VLAN database across multiple switches
E.To create collision domains

Answer: C

Correct answer: To prevent switching loops in networks with redundant switched paths

STP operates at layer 2 and prevents *switching* loops, which is the distinction the option beside it trades on - routing loops are a layer 3 problem handled by the routing protocols. It works by blocking ports until only one active path remains between any two points, keeping the redundant links in reserve.

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Which statement describes a spanning-tree network that has converged?

A.All switch and bridge ports are in the forwarding state.
B.All switch and bridge ports are assigned as either root or designated ports.
C.All switch and bridge ports are in either the forwarding or blocking state.
D.All switch and bridge ports are either blocking or looping.

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.

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What does a switch do when a frame is received on an interface and the destination hardware address is unknown or not in the filter table?

A.Forwards the switch to the first available link
B.Drops the frame
C.Floods the network with the frame looking for the device
D.Sends back a message to the originating station asking for a name resolution

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.

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In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?

A.During high-traffic periods
B.After broken links are reestablished
C.When upper-layer protocols require high reliability
D.In an improperly implemented redundant topology

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.

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If you want to disable STP on a port connected to a server, which command would you use?

A.disable spanning-tree
B.spanning-tree off
C.spanning-tree security
D.spanning-tree portfast

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.

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Which of the following statement is true?

A.A switch creates a single collision domain and a single broadcast domain. A router creates a single collision domain.
B.A switch creates separate collision domains but one broadcast domain. A router provides a separate broadcast domain.
C.A switch creates a single collision domain and separate broadcast domains. A router provides a separate broadcast domain as well.
D.A switch creates separate collision domains and separate broadcast domains. A router provides separate collision domains.

Answer: B

Correct answer: A switch creates separate collision domains but one broadcast domain. A router provides a separate broadcast domain.

Each switch port is its own collision domain, which is what removes contention, but a broadcast is flooded to every port, so the whole switch remains one broadcast domain. A router does not forward broadcasts, so each of its interfaces is a separate broadcast domain - which is why routing, or VLANs with routing between them, is what actually divides a large flat network.

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You have two switches connected together with two crossover cables for redundancy, and STP is disabled. Which of the following will happen between the switches?

A.The routing tables on the switches will not update.
B.The MAC forward/filter table will not update on the switch.
C.Broadcast storms will occur on the switched network.
D.The switches will automatically load-balance between the two links.

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.

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Layer 2 switching provides which of the following?

1.Hardware-based bridging (ASIC)

2.Wire speed

3.Low latency

4.Low cost

A.1 and 3
B.2 and 4
C.1, 2 and 4
D.All of the above

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.

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Your switch has a port status LED that is alternating between green and amber. What could this indicate?

A.The port is experiencing errors.
B.The port is shut down.
C.The port is in STP blocking mode.
D.Nothing; this is normal.

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.

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10.

You want to run the new 802.1w on your switches. Which of the following would enable this protocol?

A.Switch(config)# spanning-tree mode rapid-pvst
B.Switch# spanning-tree mode rapid-pvst
C.Switch(config)# spanning-tree mode 802.1w
D.Switch# spanning-tree mode 802.1w

Answer: A

Correct answer: Switch(config)# spanning-tree mode rapid-pvst

Two things must be right. The command is entered in global configuration mode, not privileged exec, and the keyword is rapid-pvst rather than the standard’s number - IOS names the mode after the implementation, Rapid Per-VLAN Spanning Tree, not after 802.1w.

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