Spanning Tree Protocol Operations
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Questions and Answers

What is the primary criterion used by a switch to determine which port to select when there are multiple equal-cost paths to the root bridge?

  • Lowest sender port priority
  • Lowest sender BID (correct)
  • Lowest root port cost
  • Lowest port ID
  • Which port state is employed by ports that are not designated ports or root ports?

  • Learning
  • Listening
  • Discarding (correct)
  • Forwarding
  • In the context of STP, what does the Bridge Priority configuration influence?

  • Electing the root bridge (correct)
  • Choosing designated ports
  • Setting the port state
  • Determining the root port
  • What occurs during the root bridge election process in a spanning tree?

    <p>Comparisons of internal root path costs among switches</p> Signup and view all the answers

    What does PVST+ enhance in standard spanning tree protocols?

    <p>Separate spanning tree instances for each VLAN</p> Signup and view all the answers

    Which timers are essential for STP convergence?

    <p>Hello timer, forward delay timer, max age timer</p> Signup and view all the answers

    What feature allows RSTP to provide a faster transition to the forwarding state compared to traditional STP?

    <p>PortFast functionality</p> Signup and view all the answers

    When does a port enter the forwarding state within STP?

    <p>Only if it is a root port or designated port</p> Signup and view all the answers

    What is the default Bridge ID (BID) value for a switch in a Spanning Tree Protocol (STP)?

    <p>32768</p> Signup and view all the answers

    How do switches determine which switch will become the root bridge when they have identical BIDs?

    <p>The switch with the lowest MAC address is elected as the root bridge.</p> Signup and view all the answers

    What factor determines the internal root path cost in STP?

    <p>The sum of all individual port costs to the root bridge.</p> Signup and view all the answers

    What three components are combined to determine a Bridge ID (BID)?

    <p>Bridge priority, extended system ID, and MAC address</p> Signup and view all the answers

    What happens when multiple switches have the same configured priority of 32769?

    <p>The MAC address of the switches determines the root bridge.</p> Signup and view all the answers

    How often does a switch send out BPDU frames after booting?

    <p>Every two seconds</p> Signup and view all the answers

    Which default priority is used if an administrator wishes to configure a root bridge?

    <p>32767</p> Signup and view all the answers

    In the case of a tie in the election for root bridge, what determines which switch becomes the root bridge?

    <p>The MAC address of each switch</p> Signup and view all the answers

    Which configuration allows a switch to become the root bridge in a scenario where all switches have identical priorities?

    <p>Change the port speeds to alter port costs.</p> Signup and view all the answers

    What is the default priority value for all Cisco switches?

    <p>32768</p> Signup and view all the answers

    What is the role of BPDUs in the root bridge election process?

    <p>They facilitate the exchange of root information among switches.</p> Signup and view all the answers

    What role does the BID play during the Spanning Tree Protocol (STP) operations?

    <p>It is used to elect the root bridge and port roles</p> Signup and view all the answers

    Which statement is true about the internal root path cost?

    <p>It depends on the port costs defined by the speed of each port.</p> Signup and view all the answers

    What happens after the STA elects the root bridge?

    <p>Root ports are elected next</p> Signup and view all the answers

    Which of the following values takes precedence over all other bridge priorities?

    <p>0</p> Signup and view all the answers

    During BPDU exchange, what specific information is included about the sending switch?

    <p>The BID of the sending switch</p> Signup and view all the answers

    Study Notes

    Spanning Tree Protocol Operations

    • Spanning Tree Protocol (STP) is a Layer 2 protocol used to prevent network loops
    • Switches initially declare themselves as the root bridge with their own Bridge Identifier (BID).
    • During STP, switches exchange Bridge Protocol Data Units (BPDUs) to determine a single root bridge.
    • The switch with the lowest BID becomes the root bridge.
    • BID is comprised of:
      • Priority: Default is 32768, lower is better, range is 0 to 61440 in increments of 4096
      • Extended System ID: VLAN ID added to the priority
      • MAC Address: Used to differentiate switches if priority and extended system ID are the same
    • The Root ID is the BID of the root bridge.
    • If two or more switches have the same priority, the switch with the lowest MAC address becomes the root bridge.
    • The root bridge is the reference point for path calculations.
    • BPDUs are sent every two seconds and contain:
      • BID of the sending switch
      • Root ID

    Spanning Tree Algorithm (STA)

    • The STA determines the best paths to the root bridge from all destinations in the broadcast domain.
    • The internal root path cost is the sum of all port costs along the path from a switch to the root bridge.
    • Default port costs are based on port speed, defined by IEEE 802.1D (short path cost).
    • The root port is the port closest to the root bridge in terms of internal root path cost.
    • The designated port is the port on a segment (with two switches) that has the lowest internal root path cost to the root bridge.
    • Alternate ports and backup ports are in a blocking state to prevent loops.

    Choosing a Port

    • When a switch has multiple equal-cost paths to the root bridge:
      • It uses the lowest BID, then the lowest port priority, and finally the lowest port ID to decide.

    Port States

    • Blocking: Port is disabled and discarding traffic.
    • Listening: Port is listening for BPDUs and not forwarding traffic.
    • Learning: Port is learning MAC addresses and forwarding traffic.
    • Forwarding: Port is fully operational.
    • Disabled: Port is administratively disabled and not forwarding traffic.

    STP Convergence Timers

    • Hello Timer: The time between BPDU transmissions.
    • Forward Delay Timer: The time a port waits in listening & learning state before transitioning to forwarding.
    • Max Age Timer: The time a switch considers a BPDU valid.

    STP Variations

    • Per-VLAN Spanning Tree (PVST): Each spanning tree instance is associated with a VLAN, so multiple root bridges can exist in a network.
    • Rapid Spanning Tree Protocol (RSTP): A faster version of STP with enhanced convergence.
    • PortFast: Enables ports to transition directly to the forwarding state without the listening & learning period.
    • UplinkFast: Provides a faster path to the root bridge.
    • BackboneFast: Speeds up convergence in the backbone network.
    • BPDU Guard: Prevents BPDUs from being received on ports that should not receive them.
    • BPDU Filter: Blocks BPDUs from being sent on ports that should not send them.
    • Root Guard: Prevents a port from becoming a root port.
    • Loop Guard: Detects loops and disables the offending port.

    Loop-Free Topology Steps

    • Step 1: Elect the root bridge.
    • Step 2: Elect the root ports.
    • Step 3: Elect designated ports.
    • Step 4: Elect alternate (blocked) ports.

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    Related Documents

    Cisco STP Concepts PDF

    Description

    This quiz covers the operations of Spanning Tree Protocol (STP), which is essential for ensuring loop-free network configurations. It includes details on root bridges, Bridge Protocol Data Units (BPDUs), and how switches determine their roles in the STP process.

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