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Which two network design features require Spanning Tree Protocol (STP) to ensure correct network operation? (Choose two.)

• static default routes
• implementing VLANs to contain broadcasts
• redundant links between Layer 2 switches
• link-state dynamic routing that provides redundant routes
• removing single points of failure with multiple Layer 2 switches

The correct answers to the question of which two network design features require Spanning Tree Protocol (STP) to ensure correct network operation are:

• Redundant links between Layer 2 switches
• Removing single points of failure with multiple Layer 2 switches

These two network design features inherently involve potential network loops, which can be prevented by the Spanning Tree Protocol (STP). Let’s delve into the significance of STP, its role in preventing network loops, and how these two design features rely on it for stable network operation.

Understanding Spanning Tree Protocol (STP)

The Spanning Tree Protocol (STP), defined in IEEE 802.1D, is a network protocol used on Layer 2 networks (the Data Link Layer) to prevent loop occurrences in Ethernet networks with redundant links. When multiple paths exist between switches, network loops can form. These loops can lead to various issues, such as broadcast storms, multiple frame transmission, and instability in the network due to the duplication of frames.

STP works by calculating the most efficient path in the network while disabling redundant paths, ensuring that only one active path exists between any two network devices. STP dynamically blocks the redundant links, but it can re-enable them if the primary path fails, allowing for redundancy without the risk of network loops.

1. Redundant Links Between Layer 2 Switches

Redundant links between Layer 2 switches are a common network design strategy for enhancing network reliability and availability. The idea behind redundancy is to ensure that if one link between two switches fails, there will still be a backup link in place to maintain network connectivity.

The Risk of Redundant Links Without STP

Without STP, redundant links can cause serious problems in a network. When there are multiple paths between two switches, frames can continuously circulate between the switches, causing a network loop. Here’s what can happen if STP is not present:

• Broadcast Storms: Broadcast storms occur when broadcast frames are endlessly forwarded over redundant links. Since Ethernet does not have a Time-to-Live (TTL) field like IP, broadcast frames do not expire, which means that they can keep circulating in the network, consuming all available bandwidth.
• MAC Address Table Instability: Switches use MAC address tables to determine which port to send frames to. In a network loop, the MAC address table will constantly be updated with the wrong information, as frames arrive from different ports repeatedly. This can lead to incorrect frame forwarding or frames being dropped.
• Frame Duplication: In a looped network, the same frame can be received multiple times by the destination device. This results in frame duplication, which confuses communication protocols and disrupts normal network operations.

How STP Resolves the Issue

To prevent these problems, STP identifies redundant links and puts them into a blocking state, ensuring that only one active path exists between switches. It does this by:

• Electing a Root Bridge: STP first selects a root bridge (the switch with the lowest bridge ID). All other switches calculate the shortest path to the root bridge.
• Designating Root and Non-Root Ports: For each switch, STP selects a root port, which is the port with the least-cost path to the root bridge. The non-root ports that are part of redundant links are either placed in a forwarding state (active) or in a blocking state (inactive) to prevent loops.
• Activating Redundant Links Upon Failure: STP monitors the state of active links. If an active link fails, STP will reactivate one of the blocked redundant links, ensuring that the network remains operational without any downtime.

In summary, STP is critical in managing redundant links between Layer 2 switches because it ensures that there is no looping while also keeping the backup paths ready for activation when needed.

2. Removing Single Points of Failure with Multiple Layer 2 Switches

In modern network design, removing single points of failure is crucial for building highly available networks. A single point of failure exists when a component (such as a single switch) is the only path for traffic, meaning its failure would disrupt network services for all devices connected to it. To mitigate this, network engineers deploy multiple Layer 2 switches to create alternative paths and increase network resilience.

The Risk Without STP

When multiple Layer 2 switches are deployed in the network to eliminate single points of failure, redundant links between those switches are often created. This network redundancy introduces the possibility of network loops. For example, if Switch A is connected to both Switch B and Switch C, and Switch B and Switch C are also directly connected to each other, a loop can form if traffic is sent from Switch A and forwarded across both Switch B and Switch C.

Without STP, introducing multiple Layer 2 switches to remove single points of failure can lead to the same issues discussed earlier—broadcast storms, MAC address instability, and frame duplication. The very design feature that is intended to enhance network resilience can end up causing significant operational issues if loops are not properly managed.

STP’s Role in Managing Multiple Layer 2 Switches

STP is essential for ensuring that multiple switches in a Layer 2 environment do not introduce network loops. Here’s how STP helps:

• Path Selection: STP selects the best path to the root switch, ensuring that traffic flows efficiently through the network without looping. If a switch or a link fails, STP recalculates the topology and enables a previously blocked path to maintain network connectivity.
• Blocking Redundant Paths: As with redundant links between switches, STP blocks redundant paths between multiple Layer 2 switches, preventing loops while keeping alternative paths ready to be activated in case of failure.
• Automatic Failover: One of the major advantages of STP is that it allows for automatic failover. If the primary path between switches fails, STP can unblock a backup link without any manual intervention, ensuring that network traffic continues to flow. This is particularly important when multiple switches are deployed for high availability.
• Topology Changes: When the network topology changes—such as when a new switch is added or a link is reconfigured—STP recalculates the spanning tree to adjust to the new setup. It ensures that traffic continues to flow without loops, even as the network evolves.

By implementing STP, network engineers can ensure that multiple Layer 2 switches work together to provide redundancy and eliminate single points of failure without introducing network loops. This significantly increases the resilience and reliability of the network.

Other Network Design Features That Do Not Require STP

While STP is essential for the two features mentioned above, some of the other design features listed do not rely on STP for proper operation. Here’s why:

• Static Default Routes: Static routes are configured manually and do not involve Layer 2 switching. They operate at Layer 3 (the Network Layer), directing traffic between different IP networks. STP operates at Layer 2 and is not required for static routing configurations.
• Implementing VLANs to Contain Broadcasts: VLANs are designed to segment networks and contain broadcast domains. VLANs reduce the scope of broadcast traffic but do not inherently create loops. STP can be used within a VLAN to prevent loops between switches, but the act of creating VLANs itself does not require STP.
• Link-State Dynamic Routing that Provides Redundant Routes: Link-state routing protocols like OSPF and IS-IS operate at Layer 3. They provide redundancy and route failover for IP networks but do not rely on STP. These protocols have their own loop prevention mechanisms (such as SPF algorithm and metrics) and are separate from Layer 2 loop prevention.

Conclusion

In conclusion, Spanning Tree Protocol (STP) is essential for managing two key network design features: redundant links between Layer 2 switches and removing single points of failure with multiple Layer 2 switches. These design features, while enhancing network reliability and redundancy, introduce the risk of Layer 2 network loops, which can lead to severe network problems. STP works by blocking redundant paths and preventing loops, while also ensuring that backup links can be activated in case of failure. Without STP, these network design features would compromise network stability and performance, making it a critical component of any network design that includes redundant Layer 2 paths.