Spanning Tree Protocol (STP) Interview Questions & Answers

  1. Question 1. How Is The Ttl Field Used To Prevent Indefinite Looping Of Ip Datagrams?

    Answer :

    The TTL field contains a counter value set by the source host. Each gateway that processes this datagram, decreases the TTL value by one. When the TTL value reaches zero, the datagram is discarded.

  2. Question 2. What Is The Byte Order Used For Transmitting Datagram Headers In The Tcp/ip Protocol Suite?

    Answer :

    All the datagram headers in the TCP/IP protocol suite are transmitted in the “big endian” byte order. i.e. The most significant byte is transmitted first. This is also called as “network byte order”.

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  4. Question 3. Why There Are Two Length Fields (ip Header Length, Ip Datagram Length) In The Ip Header?

    Answer :

    The size of the IP header is not fixed. Depending on the IP options present, the size of the IP header will vary. A separate field for the IP header length is added, so that the destination system can separate the IP datagram header from the payload.

  5. Question 4. When Is A Datagram Considered Undeliverable?

    Answer :

    If a datagram cannot be delivered to the destination host due to some reason, it is considered an undeliverable datagram.

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  7. Question 5. What Is The Typical Value For The Ttl Field?

    Answer :

    The typical value for a TTL field is 32 or 64.

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  9. Question 6. Which Rfc Discusses The Type Of Service (tos) Field?

    Answer :

    RFC 1349 discusses the Type Of Service (TOS) field.

  10. Question 7. What Is The Use Of The Time To Live (ttl) Field In The Ip Header?

    Answer :

    The TTL field is used to limit the lifetime of a IP datagram and to prevent indefinite looping of IP datagrams.

    Time To Live is used to limit the period of time of transmission of network technology that ba unit of data can experience before it should be discarded.

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  12. Question 8. Tell Us Is The Datagram Identifier Field Unique For Each Ip Datagram?

    Answer :

    Yes. The IP datagram identifier field is different for each IP datagram transmitted. The fragments of an IP datagram will have the same identifier value.

  13. Question 9. What Is The Spanning Tree Protocol Used For?

    Answer :

    Spanning tree protocol is used to prevent looping of frames. Unlike IP, which has a TTL value field in the packet which is decremented every time the packet passes through a router and is dropped once the value reaches 0, the frame does not have any similar field. So if a scenario arises where a frame is looping in a switched environment, the frame would not automatically time out. The spanning tree protocol is used for preventing the looping.

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  15. Question 10. How Many Root Bridges Can Be Available On A Stp Configured Network?

    Answer :

    If the priority value of the two switches are same, which switch would be elected as the root bridge

    The switch with the lowest mac-address value would be elected as the root bridge

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  17. Question 11. How Many Designated Ports Can Be Available On A Root Bridge?

    Answer :

    All ports on a root bridge is designated ports.



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  19. Question 12. Is A Generic Ethernet Frame Modified When Stp Is Configured On The Network?

    Answer :

    STP is a protocol. It has it’s own frame when configured. So it would not affect a generic ethernet frame on the network.

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  21. Question 13. Is Stp Required To Be Configured When There Is Only One Switch Involved?

    Answer :

    It is not required. STP is used to shutdown redundant links between switches to prevent loops.

  22. Question 14. Can Stp Be Configured On A Router?

    Answer :

    STP can be configured only on switches and not on routers. STP is used to prevent frame looping

  23. Question 15. What Is Extended System Id?

    Answer :

    The Extended System ID is utilized by spanning-tree to include the VLAN ID information inside 16-bit STP

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  25. Question 16. What Is Root Port?

    Answer :

    Once the Root Switch is elected, every other Switch in the network must select a single port on itself to reach the Root Switch. The port with the lowest root path cost (lowest cumulative cost to reach root switch) is elected as the root port and is placed in the forwarding state. Root Bridge will never have a Root Port.

  26. Question 17. What Is Path Cost Or Spanning Tree Path Cost Value?

    Answer :

    The Spanning Tree Cost Value is inversely proportional to the bandwidth of the link and therefore a path with a low cost value is more preferable than a path with high cost value.
    Link Bandwidth Cost Value

    • 10 Gbps 2
    • 1 Gbps 4
    • 100 Mbps 19
    • 10 Mbps 100
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  28. Question 18. How Root Bridge Is Elected?

    Answer :

    The bridge ID is used to elect the root bridge in the STP domain. This ID is 8 bytes long and includes both the priority and the MAC address of the device.

    Switch with the lowest Bridge ID is elected as the Root bridge which means Switch with the lowest priority will become Root Bridge if two or more switches have same priority than switch with lowest mac address will become Root Bridge.

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  30. Question 19. What Are Types Of Bpdu?

    Answer :

    Two types of BPDU exist:-

    1. Configuration BPDU – Used for Spanning-Tree Computation.
    2. Topology Change Notification (TCN) BPDU – Used to announce changes in the Network Topology.
  31. Question 20. What Is The Destination Mac Address Used By Bridge Protocol Data Units (bpdus)?

    Answer :

    Bridge Protocol Data Units (BPDUs) frames are sent out as at multicast destination MAC address


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  33. Question 21. What Is Bpdu?

    Answer :

    All the switches exchange information to select Root Bridge as well as for configuration of the network.

    This is done through Bridge Protocol Data Unit (BPDU). Each switch compares the parameters in the BPDU that it sends to one neighbor with the one that it receives from another neighbor.

  34. Question 22. Explain Types Of Stp Port Roles?

    Answer :

    1. Root port – The root port is always the link directly connected to the root bridge, or the shortest path to the root bridge. It is always on Non-Root Bridge.
    2. Designated port – A designated port is one that has been determined as having the best (lowest) cost. A designated port will be marked as a forwarding port. It can be on both Root Bridge & Non Root Bridge. All ports of Root Bridge are Designated Port.
    3. Forwarding port – A forwarding port forwards frames.
    4. Blocked port – A blocked port is the port that is used to prevent loops. It only listens to BPDUs. Any port other than Root port & Designated port is a Block Port.
  35. Question 23. What Are Stp Timers And Explain Different Types Of Stp Timers?

    Answer :

    STP uses three timers to make sure that a network converges properly before a bridging loop can form.

    Hello timer – The time interval between Configuration BPDUs sent by the root bridge. It is 2 seconds by default.

    Forward Delay timer – The time interval that a switch port spends in both the Listening and Learning states.

    The default value is 15 seconds.

    Max (Maximum) Age timer – Maximum length of time a BPDU can be stored without receiving an update. It can also be define as a time interval that a switch stores a BPDU before discarding it. It is 20 seconds by default.

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  37. Question 24. What Are The Different Port States?

    Answer :

    1. Disabled – A port in the disabled state does not participate in the STP.
    2. Blocking – A blocked port does not forward frames. It only listens to BPDUs. The purpose of the blocking state is to prevent the use of looped paths.
    3. Listening – A port in listening state prepares to forward data frames without populating the MAC address table. The port also sends and listens to BPDUs to make sure no loops occur on the network.
    4. Learning – A port in learning state populates the MAC address table but doesn’t forward data frames. The port still sends and receives BPDUs as before.
    5. Forwarding – The port now can send and receive data frames, collect MAC addresses in its address table, send and receive BPDUs. The port is now a fully functioning switch port within the spanning-tree topology.
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  39. Question 25. How Stp Works?

    Answer :

    STP chooses a Reference point (Root Bridge) in the network and calculates all the redundant paths to that reference point. Than it picks one path which to forward frames and blocks other redundant paths. When blocking hapeens, Loops are prevented.

  40. Question 26. What Is Stp And Redundant Links?

    Answer :

    Spanning Tree Protocol (STP) is a protocol which prevents layer 2 loops. STP enables switches to become aware of each other so that they can negotiate a Loop-Free path through network.

    In practical Scenario, Redundant links are created to avoid complete network failure in an event of failure of one link.

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  42. Question 27. What Is Spanning Tree ?

    Answer :

    Spanning-tree protocol is a protocol used in switching network to create a loop-free topology. STP is enabled by default on all VLANs on Catalyst switches. STP switches send BPDU’s (Bridge Protocol Data Units) to each other to form their topology databases. BPDU’s are sent out all ports every two seconds, are forwarded to a specific MAC multicast address: 0180.c200.0000. 

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