v1 STP Concepts
What is the purpose of redundancy in Layer 2 Switched Networks?
A) To add more physical paths for data to traverse the network
B) To create logical redundancy for eliminating single points of failure
C) To prevent disruption of network services to users
D) All of the above
The correct answer is
D) All of the above.
Redundancy in Layer 2 Switched Networks is important for eliminating single points of failure and preventing disruption of network services to users. Redundancy requires the addition of physical paths, but logical redundancy must also be part of the design. Having alternate physical paths for data to traverse the network makes it possible for users to access network resources, despite path disruption. However, redundant paths in a switched Ethernet network may cause both physical and logical Layer 2 loops, which can cause issues with network performance and reliability. To avoid these issues, network engineers can use the Spanning Tree Protocol (STP) to ensure that only one active path exists between any two network nodes.
What is the main purpose of Spanning Tree Protocol (STP)?
A) To limit the number of times a switch retransmits a Layer 2 frame
B) To recognize and eliminate endlessly looping frames in Layer 2 Ethernet
C) To limit the number of times a networking device can retransmit a packet in IPv4
D) To route packets between different networks in IPv6
Answer: B) To recognize and eliminate endlessly looping frames in Layer 2 Ethernet.
Explanation: STP was specifically designed to prevent and eliminate loops in Layer 2 Ethernet networks. Unlike Layer 3 protocols, Ethernet does not have a mechanism to limit the number of times a switch retransmits a frame. A Layer 2 loop can cause MAC address table instability, link saturation, and high CPU utilization, rendering the network unusable. STP creates a loop-free topology by blocking redundant paths and allowing only one active path between any two devices.
What is the purpose of enabling spanning tree on switches?
A) To increase network speed
B) To prevent Layer 3 loops
C) To prevent Layer 2 loops
D) To allow for Layer 2 multicasting
Answer: C) To prevent Layer 2 loops.
Explanation: Enabling spanning tree on switches is a way to prevent Layer 2 loops from occurring in a redundant network. Without spanning tree, Layer 2 loops can occur, which can cause broadcast storms, network instability, and other issues. Spanning tree allows for redundant paths to be created while maintaining a loop-free topology, preventing the issues that can occur from Layer 2 loops.
Which of the following is true regarding a loop-free topology created by STP?
A) Each switch has only one port
B) Each switch has multiple paths to the root bridge
C) Blocked ports prevent redundant paths and loops
D) Root bridge blocks all other switches
The correct answer is C) Blocked ports prevent redundant paths and loops.
When STP is enabled, it creates a loop-free topology by blocking redundant paths that could cause loops. This topology ensures that each switch has only one path to the root bridge, which is achieved by intentionally blocking some ports on the switches. The blocked ports prevent loops by making the link a non-forwarding link between two switches. This creates a tree-like structure where each switch has only one path to the root bridge, allowing for a reliable and efficient network.
What is the reason for requiring a loop-free topology in Ethernet LANs?
A) To prevent devices from communicating with each other.
B) To ensure a single path between any two devices.
C) To increase the speed of Ethernet frames.
D) To make it difficult for network administrators to manage the network.
Answer: B) To ensure a single path between any two devices.
Explanation: Ethernet LANs require a loop-free topology to ensure that there is only one path between any two devices. If there were multiple paths, Ethernet frames could continue to propagate through the network until a link is disrupted and breaks the loop, which can cause network performance problems. The spanning tree protocol (STP) is used to prevent loops by blocking redundant paths and creating a loop-free topology.
What can happen when Layer 2 loops occur in a network without STP enabled?
A) Broadcast frames loop endlessly causing the network to become unusable
B) Multicast frames are dropped and not delivered to their intended destinations
C) Unknown unicast frames are sent to all devices in the network, causing security issues
D) All of the above
Answer: D) All of the above
Explanation:
When a Layer 2 loop occurs in a network without STP enabled, broadcast, multicast and unknown unicast frames can loop endlessly, causing the network to become unusable. Broadcast frames, such as an ARP request, are forwarded out all of the switch ports, except the original ingress port, in order to ensure that all devices in a broadcast domain are able to receive the frame. However, if there is more than one path for the frame to be forwarded out of, an endless loop can result. In addition, multicast frames can also loop endlessly, and unknown unicast frames are sent to all devices in the network, which can cause security issues. These issues can cause the MAC address table on a switch to constantly change with the updates from the broadcast frames, resulting in MAC database instability, high CPU utilization, and an inability to forward frames. STP is a mechanism developed to prevent Layer 2 loops and ensure network stability.
Who invented the Spanning Tree Algorithm?
A) John Doe
B) Radia Perlman
C) Tim Berners-Lee
D) Bill Gates
Answer: B) Radia Perlman
Explanation: The Spanning Tree Algorithm was invented by Radia Perlman while working for Digital Equipment Corporation and published in her 1985 paper "An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN." The algorithm creates a loop-free topology by selecting a single root bridge, and all other switches determine a single least-cost path.
When does STP recalculate the paths and unblock the necessary ports to allow redundant paths to become active?
A) When a new host is added to the network
B) When a new switch or inter-switch link is added to the network
C) When the network experiences heavy traffic
D) When a switch's firmware is updated
Answer: B) When a new switch or inter-switch link is added to the network.
Explanation: STP recalculates the paths and unblocks necessary ports to allow redundant paths to become active when a new switch or inter-switch link is added to the network. This is because adding new switches or links can change the topology of the network, and STP needs to recalculate the paths to ensure a loop-free topology. STP does not recalculate the paths due to heavy traffic or when a switch's firmware is updated.
Which of the following statements is true about Spanning Tree Protocol (STP)?
A) STP allows for loops in Layer 2 networks.
B) STP creates a redundant Layer 2 topology.
C) STP is part of the IEEE 802.11 Wi-Fi standard.
D) IEEE 802.1D is the original IEEE MAC Bridging standard for STP.
Answer: D) IEEE 802.1D is the original IEEE MAC Bridging standard for STP.
Explanation: Spanning Tree Protocol (STP) is a network protocol that prevents loops in Layer 2 networks while allowing for redundancy. STP creates a loop-free topology by blocking redundant paths, thus ensuring that there is only one active path between any two devices. The original IEEE MAC Bridging standard for STP is IEEE 802.1D. Option A is incorrect because STP prevents loops in Layer 2 networks. Option B is partially correct because STP does create redundancy, but it also ensures a loop-free topology. Option C is incorrect because STP is not part of the IEEE 802.11 Wi-Fi standard.
Which type of frames can result in duplicate frames arriving at the destination device when sent onto a looped network?
A) Broadcast frames
B) Multicast frames
C) Unknown unicast frames
D) All of the above
Answer: C) Unknown unicast frames
Explanation: Broadcast frames are not the only type of frames that can cause a looped network to crash. Unknown unicast frames can also cause issues. When a switch doesn't have the destination MAC address in its MAC address table, it must forward the frame out all ports, except the ingress port. This can lead to duplicate frames arriving at the destination device when a loop exists in the network.
What is the purpose of Spanning Tree Protocol (STP) in a redundant Ethernet LAN topology?
A) To ensure there are multiple paths between switches
B) To create a loop-free topology
C) To reduce the amount of broadcast traffic
D) To increase network bandwidth
Answer: B) To create a loop-free topology.
Explanation: In a redundant Ethernet LAN topology, multiple paths can exist between switches, creating the potential for loops that can cause network instability and broadcast storms. STP is used to create a loop-free topology by selecting a single root bridge and then determining the least-cost path for each switch to reach the root bridge. This ensures that there is only one active path between any two switches in the network, and any redundant paths are blocked to prevent loops.
What is the purpose of path redundancy in Ethernet networks?
a) To cause a Layer 2 loop
b) To eliminate the possibility of a single point of failure
c) To create a loop-free topology
d) To prevent the propagation of Ethernet frames
Answer: b) To eliminate the possibility of a single point of failure.
Explanation: Path redundancy is an important aspect of network design to ensure high availability and eliminate the possibility of a single point of failure. This means that if a device or link fails, there is an alternate path for traffic to reach its destination without disruption. Without path redundancy, a single point of failure can cause network downtime and affect network services. However, it's important to implement loop-prevention protocols like Spanning Tree Protocol (STP) to prevent Layer 2 loops in the network.
What can cause a broadcast storm on a network?
A) A software issue on a server
B) A Layer 3 routing issue
C) A faulty network interface card (NIC) or a Layer 2 loop
D) A DNS configuration error
Answer: C) A faulty network interface card (NIC) or a Layer 2 loop
Explanation: A broadcast storm is caused by an abnormally high number of broadcast frames overwhelming the network within a specific amount of time. This can result from a hardware problem such as a faulty network interface card (NIC) or from a Layer 2 loop in the network. Broadcast storms can disable a network within seconds by overwhelming switches and end devices. Software issues on a server or a Layer 3 routing issue will not directly cause a broadcast storm. A DNS configuration error may cause issues with name resolution, but it is not related to broadcast storms.
In the spanning tree algorithm, what is the purpose of selecting a root bridge?
A) To create a loop-free topology
B) To determine the least-cost path from each switch to the root bridge
C) To select the switch with the highest bandwidth
D) To enable redundant connections between switches
Answer: B) To determine the least-cost path from each switch to the root bridge.
Explanation: The root bridge is a crucial part of the spanning tree algorithm, as it is used to create a loop-free topology. By selecting a root bridge, the switches in the network can determine a single, least-cost path from themselves to the root bridge, ensuring that there are no loops in the network. In this way, STP prevents Layer 2 loops from forming in a network with redundant connections.
What are the potential consequences of a Layer 2 loop in an Ethernet network?
A) Improved network performance
B) Increased network stability
C) MAC address table instability, link saturation, and high CPU utilization
D) Redundant paths between devices
Answer Explanation:
The correct answer is C) MAC address table instability, link saturation, and high CPU utilization.
When a Layer 2 loop occurs, the switch forwarding table can become unstable, causing broadcast and multicast traffic to flood the network, resulting in high CPU utilization on switches and end-devices. This can lead to link saturation, making the network unusable. So, path redundancy in Ethernet networks must be implemented with caution and using a protocol such as Spanning Tree Protocol (STP) to prevent Layer 2 loops.
Which of the following Layer 2 frames are forwarded in a similar way as a broadcast by the switch?
A) Multicasts
B) Broadcasts
C) Unicasts
D) None of the above
Answer: A) Multicasts
Explanation: Layer 2 multicasts, such as those used by ICMPv6 Neighbor Discovery, are forwarded in a similar way as a broadcast by the switch. This means that a Layer 2 loop can also cause problems for multicasts, just like it does for broadcasts. Therefore, it is important to prevent Layer 2 loops using mechanisms such as STP to avoid broadcast and multicast storms that can bring down the network.
Which of the following is a reason for blocking redundant paths in STP?
A) To increase network throughput
B) To prevent network loops
C) To improve network latency
D) To allow for faster convergence in case of link failure
The correct answer is B) To prevent network loops.
When there are redundant paths between network devices, it is possible for a loop to occur in the network. This can cause broadcast storms, which can disable the network. To prevent this from happening, STP blocks redundant paths, ensuring that there is only one logical path between all destinations on the network.