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STP stands for Spanning Tree Protocol. By logically blocking redundant links, the Spanning-Tree Protocol (STP) is used to prevent a loop in the Layer 2 switching network.
A layer 2 loop is the act of forwarding a single frame repeatedly among switches.
Because the layer 2 header lacks a field like the layer 3 header’s TTL field that can prevent loops Therefore, when layer frames are stuck in a loop, it never ends.
Spanning Tree Protocol (STP) finds redundant links in the switch network and places them in a blocking state.
Switches communicate with one another using STP messages known as BPDUs, or Bridge Protocol Data Units. Information that aids the switch in determining topology is contained in BPDU. BPDU is sent in every 2 Sec.
Information about changes to a network topology is announced using the Topology Change Notification (TCN) BPDU.
There must be a switch that can be used as a guide for all switches in the network in order for them to agree on a loop-free topology.
BPDU contains a field. Each switch has a distinctive identification number known as a bridge ID or switch ID, which is an 8-byte value (2 bytes for the priority field and 6 bytes for the system ID, which is the switch’s universally unique MAC address).
In a layer 2 topology, the switch with the best (numerically lower) Switch ID or Bridge ID is referred to as the root switch.
As the best path (with the lowest path cost) to the Root switch, each (non-Root) switch has exactly one Root Port.
During STP convergence, the designated port (DP) on a link receives the best (least expensive) Hello/BPDU from that link.
how long it takes for the Root Bridge to send a Configuration BPDU The default Hello,Time value is of 2 seconds.
How long a switch port stays in the Listening and Learning states The default value is 15 seconds.
The amount of time a switch keeps its best BPDU before discarding it
Spanning Tree is a common application that essentially uses a single instance calculation for all VLANs.
It is proprietary to Cisco and calculates an instance for each and every VLAN in the VLAN Spanning Tree.
redundant links between switches will be blocked, and load balancing will not be possible. Additionally, there are circumstances that could lead CST to mistakenly enable forwarding on a link that does not carry a particular VLAN while blocking other links.
When a switch port switches from blocking state to forwarding state or from forwarding state to blocking state, a topology change takes place.
Bypassing the listening and learning states, Port Fast instantly transitions an interface configured as an access or trunk port from a blocking state to the forwarding state.
The switch does not send a TCN BPDU when a fast enabled port changes state.
BPDU a guard is access port security features. When BPDU is received by a port with BPDU guard enabled, the port enters an error disable state.
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TOP 10 SPANNING TREE PROTOCOL | STP | INTERVIEW QUESTIONS & ANSWER | PART 1
Interviews for Top Jobs at STP Nuclear
Finance Interview
I interviewed at STP Nuclear
Interview
online interview asked behavioral based questions not too much specifics on personal background. very friendly and interactive, also had a personality quiz that you had to take before the interview
- tell us about a challenge
NROC Interview
I applied through college or university. The process took 5 days. I interviewed at STP Nuclear (Bay City, TX) in Apr 2021
Interview
Thorough interview and assessment. Takes all day. Skills test. Reading comprehension test. Puzzle test. Simulator test. Interview. Overall took about 5 hours to complete. Located in Bay city. They feed you lunch with us a plus.
- Give us an instance of when you were in a position of leadership.
Performance Technician Interview
I applied through an employee referral. The process took 4 weeks. I interviewed at STP Nuclear (Wadsworth, TX) in Jun 2018
Interview
I had to take a “soft skills” test online prior to the interview. The interview panel consisted of a performance technician, a supervisor, a manager and the talent acquisition lady. There was an introduction followed by 5 “soft skills” questions. No real technical questions were asked nor was there any technical discussion.
- Describe a time when your integrity was on display at a previous job.
All switches communicate with one another to choose the root bridge and to configure the network. This is done through the Bridge Protocol Data Unit (BPDU). Each switch compares the BPDU parameters it sends to a neighbor with the BPDU it receives from a different neighbor.
Spanning Tree Protocol (STP) is a protocol that avoids layer 2 loops; what are Redundant Links? Switches can negotiate a Loop-Free path through the network by becoming aware of one another thanks to STP. In a real-world scenario, redundant links are built to prevent total network failure in the event of a link failure.
Once the Root Switch has been elected, each switch in the network must choose a single port on itself to communicate with the Root Switch. The root port is chosen and set to the forwarding state using the port with the lowest root path cost (lowest cumulative cost to reach the root switch). Root bridge will never have a root port.
In order to elect the root bridge in the STP domain, the bridge ID is used. The priority and MAC address of the device are both included in this 8-byte ID. When two or more switches have the same priority, the switch with the lowest mac address is chosen to be the root bridge, which means the switch with the lowest Bridge ID becomes the root bridge.
STP uses three timers to ensure that a network converges properly before a bridging loop can form. What is an STP timer, and describe the various types of STP timers? The amount of time that has passed since the root bridge last sent a configuration BPDU. It is 2 seconds by default. The amount of time a switch port spends in both the listening and learning states is measured by the forward delay timer. The default value is 15 seconds. The maximum amount of time that a BPDU can be stored without being updated is indicated by the Max (Maximum) Age timer. It may also refer to the amount of time a switch keeps a BPDU before discarding it. It is 20 seconds by default.
A network protocol called the Spanning Tree Protocol aids in preventing network loops. It must be able to “learn” about the network in order to accomplish this, which entails being able to recognize all of the network’s connections and devices.
Reduce the time it takes for a port to switch from the blocking state to the forwarding state by using Portfast, a Cisco-specific feature. When you need to quickly connect a host to a network, like when you plug a laptop into a conference room network, this can be helpful. On a Cisco switch, you can issue the following command to enable Portfast:
Whenever a switch receives a BPDU from another switch with a lower priority, it keeps the port configuration that is currently in place. The switch will instead compare the MAC addresses of the two switches if the BPDUs are equal in priority. In contrast to the switch with the higher MAC address, the switch with the lower MAC address will maintain its current port configuration.
A separate VLAN must be created for each switch port in order to configure VLANs on a Cisco switch using STP. The switch port must then be set to trunk mode in order to carry traffic for all VLANs. Last but not least, you must turn on STP on the switch port.
A feature of the Spanning Tree Protocol (STP) called UplinkFast speeds up convergence following a topology change. This is accomplished by choosing a backup port in advance that will be used if the primary port fails. As soon as the primary port fails, the backup port is then put into a forwarding state, reducing the amount of time that is spent in the blocking state.
Regularly Placed Spanning Tree Protocol (STP) Interview Questions
Friends, welcome back to my blog series on networking interview questions. The sixth and final post in this series is this one. Prior to this, I discussed interview questions on Subnetting, Basic Networking, and Routing Parts I and II. In my previous blog, I discussed switching and VLAN, another CCNA domain. Now, in the final post of the series, I’ll discuss another crucial aspect of switching, STP interview questions and answers. With this, I’m also addressing inquiries on a crucial and fascinating subject in the switching industry, i e. VTP and EtherChannel.
So let’s start with STP – Spanning Tree Protocol
What are Redundant Links? Redundancy provides backup in runtime. Redundant links are built in switched networks to prevent total network failure in the event of a link failure.
These redundant links will create switching loops.
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A switching protocol called Spanning Tree Protocol (STP) prevents layer 2 loops caused by redundant links. STP makes it possible for switches to learn about one another so they can negotiate a loop-free path through the network.
One switch in the network is selected as the Root Bridge by Spanning Tree Protocol (STP), and it will be used as a reference point. then determines all redundant paths to that root bridge, chooses the best route for frame forwarding, and blocks all other redundant routes.
As a result of this redundant path blocking, switching loops are avoided.
What are the different port states in STP? 1. Unable to participate in the STP when a port is in the disabled state. 2. Listening: To ensure that there are no loops on the switched network, a port in the listening state sends and receives BPDU messages. The port also arranges for data frames to be forwarded without the MAT – MAC address table being filled. 3. Learning: A port in the learning state does not transmit data frames, but instead fills the MAC address table (MAT). The port is still in the listening state and continues to send and receive BPDU messages. 4. Forwarding – The port can send and receive data frames, add MAC addresses to its address table, and send and receive BPDU messages when it is in the forwarding state. Within the spanning-tree topology, this port is currently a fully functional switch port. 5. Blocking: When a port is in the blocking state, no frames are forwarded. It only listens to BPDU messages. The blocking state’s purpose is to stop the use of looped paths.
What exactly are BPDU messages? All switches transmit data to determine the root bridge and to configure the network. The messages known as Bridge Protocol Data Unit (BPDU) are used to accomplish this. Each switch compares the parameters in the BPDU messages it sends to each of its neighbors with the messages it receives from each of those neighbors.
Explain the three timers that STP uses: the Hello, Forward Delay, and Maximum Age timers. These timers ensure that a switched network converges correctly before redundant links can cause a bridging loop to form. The amount of time that has passed since the root bridge sent a configuration BPDU message. It is 2 seconds by default. The amount of time a switch port spends in the Listening and Learning states before switching to the forward state is measured by the forward delay timer. The default value is 15 seconds. The maximum amount of time that a BPDU can be stored before being discarded is indicated by the Max (Maximum) Age timer. It can also be described as the amount of time between updates that a switch stores BPDU data. By default, it is 20 seconds.
What are the various STP port roles, and why do they exist? Root port – The root port is always on the non-root bridge. There is only ever one port on a link that is directly connected to the root bridge or has the shortest path to the root bridge. It is always in the forward state. A designated port may be located on either the root bridge or a non-root bridge. Also, all ports of the root bridge are designated port. The port with the best (lowest) cost to the root bridge is a designated port. It will be marked as a forwarding port. A forwarding port is able to send and receive information. e. forwards frames. Blocked port: The port used to avoid switching loops is known as a blocked port. It is also called a Non-designated port. It only listens to BPDU messages. Every port aside from root and the designated port is blocked.
BPDUs (Bridge Protocol Data Units) frames are transmitted as multicast frames; what is the destination MAC address used by BPDUs? The destination MAC address for BPDUs is 01:80:c2:00:00:00.
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There are two different types of BPDUs, which are as follows: 1) Configuration, which is used for spanning-tree protocol (STP) evaluation 2) Topology Change Notification (TCN): This TCN BPDU notifies the network of any changes to the topology.
What are the steps of STP working? The steps of STP working are as follows:
- Calculate Path cost and Root Path cost for every bridge. (Optional).
What is the Bridge ID? It has a length of 8 bytes. It contains the device’s MAC address in addition to the priority. This bridge ID is utilized in the STP domain to choose the root bridge.
The bridge with the lowest Bridge ID is chosen to serve as the root bridge in the STP domain. If two or more switches have the same priority, the switch with the lowest mac address will choose to be the root bridge, so the switch with the lowest priority will elect to be the root bridge.
The Spanning Tree Path Cost Value is inversely correlated to the link’s bandwidth, meaning low cost represents high bandwidth, so it is also known as the Path, Link, or STP Path Cost value. Consequently, a route with a low cost value is preferable to one with a high cost value.
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When the Root bridge is chosen, all other Switches are connected to the e. To connect to the Root bridge, the Non-Root bridge in the STP domain must choose its single port. The root port is designated as the port with the lowest Root path cost. It is always in the forwarding state.
A root port is only present on non-root bridges, and a root port will never exist on a root bridge.
What is VLAN Trunking Protocol (VTP)? Cisco switches use their own proprietary protocol called VTP (VLAN Trunking Protocol) to exchange VLAN information. With switches inside the same VTP domain, this protocol is used to coordinate VLAN information such as VLAN ID, VLAN Name, the creation of new VLANs, and the deletion of existing VLANs.
There are three different working modes for VTP (VLAN Trunking Protocol), and they are as follows:
What conditions must be met before VTP messages can be sent between different switches? The same VTP domain name needs to be used across all switches. 2. A switch should be set up as a VTP client, server, or transparent mode. 3. All switches in the VTP domain should have the same VTP versions. 4. On each switch in the VTP domain, the VTP password must be the same. 5. The connection between the switches needs to be set up as a trunk link.
What is VTP Pruning? The VTP pruning feature of Cisco switches will stop sending VLAN update information traffic to that specific switch’s trunk link if no VLAN updates are required for that switch. This prevents unnecessary flooding of frames. VLAN 1 (the default VLAN) cannot be pruned because it is an administrative VLAN.
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EtherChannel is a port-channel structural design or port link accretion technology that is primarily used on Cisco switches. EtherChannel allows assemblage of a number of real Ethernet links. By putting them together, it creates a logical Ethernet link that guarantees the provision of fault-tolerance and fast links. EtherChannel can be established between switches, routers and servers. When forming an EtherChannel, a maximum of 8 Fast Ethernet or 8 Giga Ethernet ports may be connected together.
What are the available mechanisms for configuring EtherChannel?
There are 3 mechanisms you can choose to configure EtherChannel:
Check out my blog on EtherChannel for comprehensive information.
Friends, this concludes my blog series on networking interview questions. I hope you now have a ton of questions and their responses with you so that you can confidently approach any networking profile interview.
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Author:- Sumaiyya Suhail Bagwan Department Name:- Networking Designation:- Technical Trainer
FAQ
What is STP interview questions?
- What is STP and Redundant Links? …
- How STP works? …
- What are the different port states? …
- What is a STP Timer and what are its various types?
- Explain types of STP Port Roles? …
- What is BPDU?
How do I prepare for STP interview?
You must be knowledgeable in your field, aware of what to anticipate, and equipped to respond to any inquiries. Think about what could be asked by the station descriptions. Read up on any recent scientific advancements in your field and the clinical aspect of your specialty.
How STP works step by step?
A Layer 2 network protocol called Spanning Tree Protocol (STP) is used to stop loops in a network topology. STP was developed to prevent the issues that occur when computers exchange data on a LAN with redundant paths.
