Advanced STP Concepts Every CCNA Candidate Should Know

Spanning Tree Protocol, often abbreviated as STP, plays a pivotal role in modern Ethernet networks, serving as a guardian of loop-free data paths. Its presence in network design isn’t just beneficial; it’s imperative. Designed under the IEEE 802.1D standard, STP operates silently in the background, forming an invisible scaffold that upholds network stability and prevents catastrophes such as broadcast storms and incessant frame duplication.

Ethernet networks, particularly those with redundant paths, are susceptible to loops, which can disrupt communication and degrade performance dramatically. In a topology lacking loop prevention, a single broadcast frame might circulate endlessly, consuming bandwidth and incapacitating switches. The ingenuity of STP lies in its ability to maintain a single active route between devices while leaving alternate paths on standby, poised to spring into action should the primary route fail.

Understanding the Logical Tree Structure

At its core, STP constructs a logical hierarchy that mimics the branches of a tree. It does this not by physically disabling cables or interfaces but by orchestrating port states on network switches. Certain ports are elected to forward traffic, while others are relegated to a passive role to preserve order. This elegant choreography ensures that each frame takes the most efficient path to its destination without circling back on itself.

The logical framework developed by STP encompasses both strategy and precision. Switches within the network exchange Bridge Protocol Data Units, known colloquially as BPDUs. These specialized messages enable switches to share identity and path cost information, collaboratively determining the optimal structure of the network. The exchange of BPDUs is continuous, ensuring that the topology remains updated and responsive to changes.

The Role of the Root Bridge

Every STP topology revolves around a central figure—the Root Bridge. This switch is not selected arbitrarily. Instead, switches undergo a democratic process wherein each announces its Bridge ID, a composite value derived from its MAC address and configurable priority. The switch boasting the lowest Bridge ID ascends to the position of Root Bridge.

The significance of the Root Bridge extends beyond its symbolic status. It becomes the benchmark for all path calculations. Every switch in the topology evaluates its proximity to the Root Bridge by assessing cumulative path costs, which are numerical values representing the expense of traversing a specific link. These evaluations dictate which ports become Root Ports, Designated Ports, or Blocked Ports.

Critical Port Roles in STP

Root Ports are the conduits through which non-root switches communicate with the Root Bridge. Each switch selects one Root Port—the one offering the path of least resistance, metaphorically speaking. These ports are vital arteries in the STP circulatory system.

Designated Ports, in contrast, are selected on a per-segment basis. They offer the most efficient path from that segment to the Root Bridge and are entrusted with forwarding traffic from their segment.

Blocked Ports function as strategic reserves. Though physically connected and capable of forwarding data, they are placed in a dormant state to prevent loops. This latent potential is not wasted; these ports can become active should the network topology change due to link failure or switch removal.

How STP Prevents Loops

The meticulous prevention of loops lies in STP’s intelligent port state transitions. A newly connected switch doesn’t begin forwarding traffic immediately. Instead, its ports cycle through Listening and Learning states, during which they absorb BPDU information and build a MAC address table. Only after proving their safety and efficiency do ports graduate to the Forwarding state.

This systematic approach introduces a delay but serves a higher purpose. It ensures that only ports which do not threaten the loop-free nature of the topology are permitted to forward traffic. Consequently, redundancy is preserved without compromising stability.

The Importance of Convergence

Convergence refers to the network’s ability to recalibrate itself following a change in topology. This might be the result of a cable being unplugged, a switch failing, or a new device being introduced. During convergence, switches re-evaluate their BPDU data, potentially re-electing the Root Bridge and redistributing port roles.

The speed and accuracy of this process are paramount. An efficient convergence process means minimal disruption. In traditional STP, convergence might take upwards of 50 seconds, an eternity in network terms. Nonetheless, the foundational version of STP emphasizes consistency and integrity over speed, ensuring that network loops never creep back into existence.

Bridge ID and Its Influence

The Bridge ID stands at the heart of the election process. It combines two critical elements: the Bridge Priority and the MAC address. While the MAC address is hardcoded and unique, the Bridge Priority is configurable, giving network administrators a measure of control over which switch is more likely to be elected as the Root Bridge.

By manipulating the priority value, a preferred switch can be nudged into the Root Bridge role, thereby sculpting the STP topology to align with network design goals. This proactive configuration technique is widely used in enterprise environments where network predictability is essential.

STP Timers and Their Functions

Timers play a subtle yet significant role in the workings of STP. Three primary timers dictate its tempo:

The Hello Timer determines how frequently BPDUs are sent. This continual communication ensures that all switches remain informed.
The Forward Delay Timer governs the duration a port spends in the Listening and Learning states.
The Max Age Timer defines how long a BPDU is considered valid before it is discarded.

Proper calibration of these timers is crucial. If set too low, the network may become overly reactive, responding to transient changes that don’t warrant intervention. If too high, the network may respond sluggishly to genuine issues.

Redundancy and Resilience

One of the most advantageous characteristics of STP is its ability to harness redundancy without succumbing to its drawbacks. By retaining alternate paths in a blocked state, STP ensures that these paths are ready for deployment should the need arise. This dormant redundancy serves as a form of insurance, granting the network a capacity for self-healing.

This intrinsic resilience is not merely theoretical. In live environments, link failures are not uncommon. Cables can be damaged, switches can lose power, and configurations can change. STP mitigates the potential chaos by automatically recalculating a new loop-free topology, ensuring continuity.

Advanced Mechanics and Operational Dynamics of STP

Understanding the underlying principles of Spanning Tree Protocol forms only the foundation. Delving deeper into its advanced mechanics reveals a labyrinth of processes that ensure network dependability and facilitate the seamless handling of changes within Ethernet topologies. These operational dynamics distinguish STP as a formidable protocol in the arsenal of any network engineer.

The Anatomy of BPDU Communication

Bridge Protocol Data Units are more than just information packets; they are the language spoken between switches in a network governed by STP. Each BPDU contains a trove of data including the sender’s Bridge ID, the Root Bridge ID as perceived by the sender, the path cost to the Root Bridge, and various timer values.

The transmission of BPDUs is governed by precision. The Root Bridge emits BPDUs at intervals defined by the Hello Timer. These BPDUs cascade through the network, with each switch amending and forwarding them to downstream neighbors. This flow of information is akin to a neural network constantly pulsing with updates, sustaining the integrity of the topology.

When a switch receives a superior BPDU—that is, a BPDU indicating a better path to the Root Bridge—it recalibrates its port roles accordingly. This agility in response ensures that the most efficient network layout is always in place.

The Role of Path Cost in Decision Making

Path cost functions as the currency of STP decision making. It quantifies the desirability of a path, with lower costs representing more optimal routes. Cost values are not arbitrary; they are determined by the speed of the link. For example, a 100 Mbps link has a higher cost than a 1 Gbps link, making the latter more attractive for Root Port selection.

This metric-driven approach allows switches to make rational choices based on quantifiable parameters. In networks where multiple links of different capacities coexist, path cost serves as a discriminating factor, ensuring traffic flows through the most capable routes.

Port States and Their Significance

Ports participating in STP operate in one of several states, each with distinct characteristics and responsibilities:

Blocking: The default state that prevents loops by disallowing frame forwarding.
Listening: The port prepares to transition to a more active state by receiving BPDUs but not learning MAC addresses.
Learning: In this phase, the port begins building the MAC address table without forwarding frames.
Forwarding: Fully operational, the port sends and receives frames.
Disabled: A state used when a port is manually shut down or fails to meet operational standards.

Each port must traverse the Blocking, Listening, and Learning states before reaching Forwarding. This graduated progression ensures network stability by minimizing the risk of premature frame forwarding that could trigger loops.

Topology Change Notification

Network topologies are seldom static. Devices join, leave, or reposition frequently, especially in dynamic enterprise environments. STP handles these changes with a methodical mechanism known as Topology Change Notification (TCN).

When a switch detects a change—such as a port transitioning to the Forwarding state—it generates a TCN BPDU and sends it toward the Root Bridge. The Root Bridge, upon receipt, disseminates the change notification across the network. This prompts all switches to shorten their MAC address aging time, ensuring obsolete paths are purged quickly.

This mechanism enhances the network’s agility. Rather than clinging to outdated address mappings, switches remain adaptive and responsive to real-time changes, maintaining optimal communication pathways.

Manual Control and Designation

While STP is largely autonomous, network architects possess tools to steer its behavior. Adjusting the Bridge Priority value allows for influence over Root Bridge selection. By setting a lower priority on a preferred switch, administrators can ensure its dominance in the election process.

This level of control is invaluable in scenarios where predictable performance is paramount. For instance, in a data center, ensuring that a core switch assumes the Root Bridge role can streamline traffic flows and reduce unnecessary delays.

Furthermore, administrators can manipulate port costs to influence Root Port and Designated Port selection. This granular control allows for intentional traffic engineering, optimizing bandwidth utilization and enhancing fault tolerance.

Resilience Through Redundancy

Redundancy is both a strength and a challenge in network design. STP’s brilliance lies in its ability to embrace redundant links without succumbing to their pitfalls. By activating only the most efficient paths and keeping others in reserve, STP weaves redundancy into the fabric of resilience.

Should a primary link fail, STP springs into action, recalculating the topology and activating previously blocked paths. This self-healing capability transforms network infrastructure into a robust, adaptive organism, capable of enduring disruptions with minimal impact.

Troubleshooting STP with Diagnostic Tools

Effective troubleshooting requires more than observation; it demands interpretation. STP offers a suite of diagnostic tools that illuminate its internal workings. The most frequently utilized is the command that reveals the current spanning tree status, detailing Root Bridge identity, port roles, and path costs.

Armed with this data, engineers can identify misconfigurations, such as unintended Root Bridges or asymmetric path costs. Further analysis of BPDU traffic can expose issues like inconsistent port states or erratic convergence behavior.

Another invaluable resource is the ability to observe port state transitions. By tracking how and when ports change roles or states, anomalies can be detected and addressed promptly. These insights enable swift resolution, ensuring network reliability.

Convergence and Its Impact

Convergence time, the duration required for the network to stabilize following a change, directly influences performance. In traditional STP, convergence can span up to 50 seconds, a delay that, while cautious, may not suit environments demanding rapid responsiveness.

Understanding this limitation is vital. It underscores the necessity of alternative protocols in time-sensitive networks. Nonetheless, STP’s measured approach offers predictability and minimizes the risk of oscillation or transient loops.

Considerations for Network Design

Designing with STP in mind involves balancing redundancy with predictability. Network architects must consider link speeds, expected traffic patterns, and the criticality of certain paths. Selecting the Root Bridge strategically and fine-tuning port costs contribute to a topology that aligns with organizational goals.

In hierarchical designs, placing the Root Bridge in the core layer can simplify traffic flow and reduce complexity. Meanwhile, access and distribution layers can be configured to complement this structure, ensuring that STP supports rather than obstructs efficient communication.

MAC Address Aging and Forwarding Tables

STP indirectly affects the operation of MAC address tables. During topology changes, MAC addresses learned on affected ports may become invalid. The aging time, typically 300 seconds, determines how long these addresses are retained.

Upon a topology change, STP reduces the aging time temporarily. This acceleration ensures that stale entries are purged, and new, accurate mappings can be established. This mechanism preserves the fidelity of forwarding decisions, contributing to network agility.

Avoiding Common Pitfalls

Misconfigurations can compromise the effectiveness of STP. Setting identical priorities across all switches may lead to unintended Root Bridge elections. Inconsistent path costs can cause suboptimal route selection. Ignoring port roles during design can result in congestion or asymmetrical traffic flow.

Meticulous planning and regular audits can avert these issues. Engineers should document STP configurations, maintain consistent timer values, and periodically verify port states. This proactive approach fosters a stable and high-performing network environment.

Variants of STP and Their Strategic Applications

Spanning Tree Protocol has undergone various refinements since its inception, evolving to meet the escalating demands of contemporary Ethernet infrastructures. These refinements have birthed several STP variants, each bringing forth its own improvements, particularly in convergence speed, scalability, and VLAN management. Understanding these alternatives is crucial for optimizing network design and ensuring seamless performance across multifaceted environments.

The Emergence of Rapid Spanning Tree Protocol (RSTP)

Rapid Spanning Tree Protocol emerged as a natural progression from traditional STP, offering swifter convergence times and greater agility. Defined under IEEE 802.1w, RSTP reimagines the mechanisms of its predecessor without abandoning the foundational principles. By reducing the time needed for port transitions, RSTP significantly enhances responsiveness to topological shifts.

One of the most salient enhancements in RSTP is its treatment of port roles and states. Unlike the traditional model that traverses several intermediary stages, RSTP streamlines port transitions by eliminating the Listening state and accelerating the transition from Learning to Forwarding under certain conditions.

Moreover, RSTP introduces new port roles such as Alternate and Backup, which facilitate more dynamic redundancy. These roles allow non-designated ports to become active more quickly when needed, reducing downtime during reconfiguration.

RSTP Port Roles and States

The RSTP framework expands the STP role-based architecture with the following distinctions:

Root Port: The closest path to the Root Bridge.
Designated Port: The forwarding port for a specific segment.
Alternate Port: A backup path to the Root Bridge, intended to take over if the Root Port fails.
Backup Port: A backup for the Designated Port on the same segment.

Each of these roles is tightly coupled with the port’s state, which can be one of three primary conditions: Discarding, Learning, or Forwarding. These concise states replace the five-step progression found in traditional STP, resulting in swifter reactions and more intuitive diagnostics.

Advantages of Deploying RSTP

The benefits of RSTP extend well beyond convergence speed. It includes improved fault detection, reduced timer dependency, and more deterministic behavior. This efficiency is especially valuable in environments with frequent link changes, such as campus networks or enterprise-grade switching backbones.

Additionally, RSTP fosters a proactive topology by leveraging point-to-point links to exchange BPDUs more frequently. This leads to better coordination and quicker detection of topology changes, effectively minimizing the window of potential disruption.

Introducing Multiple Spanning Tree Protocol (MSTP)

As network complexity grew, especially with the proliferation of VLANs, the need for a more scalable solution became evident. Multiple Spanning Tree Protocol, standardized under IEEE 802.1s, was conceived to allow multiple spanning trees to coexist, each mapped to specific VLANs.

This mapping provides fine-grained control over traffic distribution, enabling network engineers to optimize resource utilization and load balancing across multiple paths. MSTP allows for the bundling of VLANs into instances, each with an independently calculated spanning tree, thereby reducing overhead and improving performance.

Internal Structure of MSTP

MSTP introduces the concept of MST Regions. These are logical constructs within the network where VLANs are associated with specific Spanning Tree Instances (STIs). Each region maintains a single Internal Spanning Tree (IST) for communication between instances and other regions.

Within each region:

VLANs are grouped into a defined number of instances.
Each instance behaves like a separate STP process.
The IST ensures that BPDUs are exchanged between regions, maintaining a unified topology.

This stratified design ensures scalability without sacrificing the core loop-prevention function.

Strategic Benefits of MSTP

MSTP’s architecture presents multiple operational advantages. By allowing VLANs to be grouped and managed collectively, it reduces the computational load on network devices. Unlike PVST+, which runs a separate STP process for each VLAN, MSTP’s instance-based model consolidates these into fewer processes.

The practical result is a network capable of handling greater VLAN density with improved efficiency. Additionally, MSTP enables engineers to engineer traffic flows more deliberately, assigning higher priority paths to latency-sensitive services such as voice and video.

Comparing STP, RSTP, and MSTP

While each variant shares a common heritage, their differences underscore distinct use cases:

Traditional STP is known for stability but suffers from sluggish convergence.
RSTP offers rapid reconfiguration and simplified role transitions.
MSTP delivers scalability and control for VLAN-rich topologies.

Selecting the appropriate variant requires evaluating the network’s scale, performance expectations, and administrative goals. Smaller networks may function adequately with STP, but larger, more dynamic environments benefit substantially from the responsiveness of RSTP or the modular design of MSTP.

VLAN-aware Spanning Tree Variants

Network environments that incorporate VLANs introduce additional complexity. Per-VLAN Spanning Tree Plus (PVST+) was designed to accommodate these scenarios by allowing each VLAN to maintain its own independent spanning tree.

While PVST+ enhances flexibility and load balancing, it introduces more processing overhead. Each VLAN operates a separate STP instance, which can tax switch resources as VLAN count escalates. This is where MSTP excels, combining scalability with VLAN awareness.

Rapid PVST+, another innovation, fuses the benefits of RSTP with the VLAN-specific capabilities of PVST+, enabling fast convergence per VLAN. However, it shares the same scalability concerns due to its per-VLAN instance model.

Transitioning Between STP Variants

Migrating from one STP variant to another requires meticulous planning. Compatibility between switches is crucial, especially during the interim phase. For instance, mixing traditional STP and RSTP requires that RSTP devices fall back to STP behavior when communicating with legacy switches.

Best practices for transitions include:

Upgrading edge switches first to minimize disruption.
Ensuring consistent instance mappings in MSTP configurations.
Testing convergence behavior in a controlled environment before deployment.

A phased approach helps preserve network continuity while embracing the advantages of newer protocols.

STP Optimization Techniques

Whichever variant is deployed, network performance can benefit from several optimization techniques. Adjusting Bridge Priorities allows engineers to dictate Root Bridge elections, thereby shaping traffic paths proactively.

Port cost adjustments are another valuable lever. By artificially increasing or decreasing the cost of specific links, traffic can be steered away from or toward particular routes. This technique is particularly useful in creating preferred backup paths or enforcing bandwidth hierarchies.

Implementing PortFast on access ports connected to end devices expedites port activation by bypassing intermediary states. This reduces the startup delay experienced by endpoint devices and mitigates potential user dissatisfaction.

BPDU Guard and BPDU Filter provide additional layers of security. They prevent unauthorized devices from influencing STP calculations, preserving the intended topology and deterring potential misconfigurations or attacks.

Real-World Applications and Considerations

In metropolitan-area networks and campus-wide infrastructures, STP variants play an essential role in maintaining service reliability. RSTP’s rapid reconfiguration is invaluable in environments with frequent link transitions, while MSTP’s instance management supports intricate traffic segmentation across VLANs.

In data centers, where performance and uptime are paramount, the careful implementation of MSTP ensures that high-priority services remain uninterrupted even during topology changes. Meanwhile, access-layer switches can be optimized using PVST+ or Rapid PVST+ for their intuitive VLAN management.

Careful calibration of timers, cost metrics, and priority values enables engineers to sculpt behavior to precise organizational needs. With proper design, even complex topologies can remain agile and robust, adapting fluidly to dynamic conditions.

STP Best Practices, Troubleshooting, and Real-World Implementation

The practical implementation of Spanning Tree Protocol transcends theoretical understanding, demanding a blend of precision, strategy, and foresight. While knowing the variants and mechanics is essential, the true value of STP is realized when it is deftly deployed and meticulously maintained.

Embracing Best Practices in STP Configuration

Deploying STP with foresight ensures that redundancy, stability, and performance coexist harmoniously. Proper configuration is the cornerstone of reliability. A strategic first step involves designating the Root Bridge intentionally. Instead of relying on arbitrary elections, network engineers should assign Bridge Priorities that favor devices in the core layer, thus anchoring the topology predictably.

Equally important is the tuning of port costs. By assigning appropriate values based on link capacity and desired path selection, administrators can influence route decisions. This ensures that high-bandwidth links carry critical traffic, while less efficient paths remain in reserve.

For access ports—those connected to end-user devices—enabling PortFast is indispensable. It eliminates delays caused by port state transitions, allowing devices to connect and communicate without unnecessary latency. However, it must be used with care, as improper use on trunk links could introduce vulnerabilities.

Security Measures Within STP

Spanning Tree Protocol, if left unguarded, can be exploited to manipulate network paths or cause instability. To safeguard its operation, two critical features should be considered: BPDU Guard and BPDU Filter.

BPDU Guard protects the network by shutting down ports that receive unexpected BPDUs. This is particularly useful on access ports, where BPDUs are not expected. It prevents rogue devices from injecting misleading data into the STP domain.

BPDU Filter, on the other hand, suppresses the sending and receiving of BPDUs on designated ports. This is useful in isolated segments where STP participation is unnecessary or could be disruptive. However, its application requires careful planning to avoid inadvertently disabling loop protection.

Diagnosing STP Issues

Even the most meticulously designed networks can encounter anomalies. Effective troubleshooting of STP involves a systematic approach beginning with the assessment of the current topology. Commands that reveal Root Bridge identity, port roles, and state transitions serve as the diagnostic baseline.

Signs of trouble may include unexpected Root Bridge elections, frequent topology changes, or inconsistent port roles. These symptoms often point to configuration mismatches, such as equal Bridge Priorities or misaligned path costs. Reviewing BPDU transmissions can also unearth hidden issues, such as unauthorized devices participating in the election process.

Another diagnostic method involves tracking MAC address table behaviors. A sudden flood of unknown unicast frames, or frequent MAC address movement between ports, may indicate loop formation. Identifying and isolating the affected segment can prevent further disruption.

Interpreting STP Timers

STP timers influence how quickly the network reacts to changes. The three core timers—Hello, Forward Delay, and Max Age—can be adjusted to refine responsiveness. However, arbitrary changes can cause instability.

The Hello Timer governs how frequently the Root Bridge sends BPDUs.
The Forward Delay controls how long a port remains in transitional states.
The Max Age determines how long a switch retains outdated BPDU information.

Optimal values depend on network size, topology complexity, and redundancy goals. Inappropriately short timers can lead to flapping, while excessive durations may delay convergence.

Real-World Implementation Techniques

In enterprise environments, a hierarchical design typically consists of core, distribution, and access layers. Assigning Root Bridge status to a core switch centralizes path computation and enhances consistency. The distribution layer can serve as an intermediary, while access switches benefit from rapid convergence via PortFast.

In data centers, where downtime carries high costs, deploying MSTP ensures that VLANs are logically grouped and that redundant paths are efficiently managed. Traffic-sensitive applications, such as VoIP or financial systems, can be prioritized using tailored port costs and instance configurations.

Service providers operating wide-area or metropolitan networks often rely on RSTP due to its responsiveness and simplified management. Its ability to detect and recover from link failures quickly maintains service continuity across vast infrastructures.

Contingency and Failover Planning

Redundancy is a double-edged sword. While it enables failover, it must be balanced with control to avoid erratic behavior. STP provides mechanisms to ensure that alternate paths remain dormant yet ready.

Implementing loop guard protects against the unexpected transition of blocked ports to forwarding if BPDUs stop arriving. This preemptive defense prevents the formation of loops during network instability.

Root Guard further refines control by preventing unauthorized devices from assuming the Root Bridge role. Applied to ports leading to access switches, it ensures that the core topology remains undisturbed by unintended re-elections.

Monitoring and Maintenance

Ongoing monitoring is critical to sustaining STP effectiveness. Tools that visualize the topology, log BPDU activity, and highlight port state changes offer real-time insights into network health.

Regular audits can identify creeping misconfigurations, such as newly added switches with default priorities or altered port settings. Documenting the intended design and comparing it periodically with the live environment reveals discrepancies that may affect performance or resilience.

Proactive maintenance also includes firmware updates and reviewing vendor-specific enhancements. Many modern switches support proprietary features that supplement STP capabilities, such as faster re-convergence or automatic loop detection.

Integrating STP with Other Protocols

In complex environments, STP often coexists with other protocols. Link Aggregation Control Protocol (LACP) bonds multiple links into a single logical connection, distributing traffic and enhancing throughput. STP recognizes these bundles as a single path, simplifying topology calculation.

Additionally, STP can be complemented by routing protocols in Layer 3 switching environments. While STP governs Layer 2 loops, routing protocols ensure efficient path selection at Layer 3. Harmonizing their operations avoids conflicts and supports comprehensive traffic management.

Virtual LANs further complicate the STP landscape. Integrating PVST+ or MSTP allows engineers to align spanning tree instances with VLAN distribution. This modular approach enables service-specific path control and facilitates performance tuning.

Adaptability and Evolution

Although STP has existed for decades, its relevance persists due to its adaptability. As networks expand in scope and complexity, the protocol has evolved, absorbing new functions and accommodating modern demands.

Engineers must embrace this evolution by staying informed of advancements and continuously refining their deployment strategies. Emerging technologies, such as Software-Defined Networking, may alter the landscape, but the principles of redundancy, control, and failover remain foundational.

By anticipating change and applying the wisdom of experience, STP continues to serve as a vital pillar in network architecture.

Conclusion

Mastering the real-world application of Spanning Tree Protocol requires more than knowledge of its mechanisms. It demands strategic foresight, proactive security, vigilant monitoring, and a readiness to adapt. From configuring Root Bridges and port costs to safeguarding against loops and unauthorized changes, each decision shapes the behavior of the network.

The tools, techniques, and practices explored here form a comprehensive guide for implementing STP with confidence. Whether managing a compact office network or a sprawling data center, these insights empower network professionals to uphold stability, efficiency, and resilience in every layer of their infrastructure.