What is STP and how does it prevent network loops?
Theme: Routing and Switching Role: Network Engineer Function: Technology
Interview Question for Network Engineer: See sample answers, motivations & red flags for this common interview question. About Network Engineer: Designs and maintains computer networks. This role falls within the Technology function of a firm. See other interview questions & further information for this role here
Sample Answer
Example response for question delving into Routing and Switching with the key points that need to be covered in an effective response. Customize this to your own experience with concrete examples and evidence
- STP Overview: STP stands for Spanning Tree Protocol. It is a network protocol used to prevent loops in Ethernet networks
- Loop Prevention Mechanism: STP prevents network loops by creating a loop-free logical topology
- Root Bridge Election: STP elects a root bridge, which becomes the central point of the network. All other switches are designated as either root or non-root bridges
- Bridge Protocol Data Units (BPDU): STP uses BPDU messages to exchange information between switches. BPDU messages contain information about the root bridge, path cost, and bridge priority
- Path Cost Calculation: STP calculates the path cost for each switch port based on the bandwidth of the link. The port with the lowest path cost is selected as the root port
- Port Roles: STP assigns different roles to switch ports, including root port, designated port, and blocking port
- Port States: STP defines different port states, including blocking, listening, learning, forwarding, and disabled
- Loop Detection & Blocking: STP detects network loops by monitoring the BPDU messages. If a switch receives a BPDU with a lower bridge ID, it will block the port that received the BPDU to prevent loops
- Topology Changes: STP handles topology changes by recalculating the spanning tree and updating the port roles and states accordingly
- Rapid Spanning Tree Protocol (RSTP): RSTP is an enhanced version of STP that provides faster convergence and supports faster link failover
- Multiple Spanning Tree Protocol (MSTP): MSTP allows for the creation of multiple spanning trees within a network, providing better scalability and load balancing
Underlying Motivations
What the Interviewer is trying to find out about you and your experiences through this question
- Technical knowledge: Assessing the candidate's understanding of STP and its role in preventing network loops
- Problem-solving skills: Evaluating the candidate's ability to identify and resolve network loop issues
- Experience with network protocols: Determining the candidate's familiarity with STP and its implementation in real-world scenarios
- Troubleshooting skills: Assessing the candidate's capability to troubleshoot and mitigate network loop problems
Potential Minefields
How to avoid some common minefields when answering this question in order to not raise any red flags
- Lack of understanding: Providing a vague or incorrect definition of STP or its purpose
- Inability to explain network loops: Failing to describe how network loops occur and the problems they can cause
- Limited knowledge of STP: Not being able to explain the different versions of STP or its key features
- No mention of prevention methods: Neglecting to mention STP's role in preventing network loops or how it achieves this
- Failure to discuss STP's impact on network performance: Not addressing how STP can affect network performance or potential drawbacks of its implementation