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Difficulty: MediumSpanning Tree Protocol Operations

A network engineer is configuring standard IEEE 802.1D Spanning Tree Protocol (STP) across a enterprise switch topology. Currently, all switches in the network operate using the default bridge priority of 3276832768. The engineer needs to explicitly configure Switch-1 to guarantee it becomes the primary Root Bridge, and Switch-2 to serve as the designated backup Root Bridge if Switch-1 fails. Which combination of bridge priority settings should the engineer assign to achieve this design?

  1. Assign a bridge priority of 40964096 to Switch-1 and 81928192 to Switch-2.Answer
  2. B
    Assign a bridge priority of 6144061440 to Switch-1 and 4915249152 to Switch-2.
  3. C
    Assign a bridge priority of 40964096 to Switch-1 and 4096040960 to Switch-2.
  4. D
    Assign a bridge priority of 3276932769 to Switch-1 and 3277032770 to Switch-2.

Answer

Assign a bridge priority of 4096 to Switch-1 and 8192 to Switch-2.
In Spanning Tree Protocol (STP), the Root Bridge is elected based on the lowest Bridge ID, which consists of a 2-byte Bridge Priority and a 6-byte MAC Address. Because the default priority on switches is 3276832768, assigning a lower priority value such as 40964096 to Switch-1 guarantees it will win the root election. Assigning 81928192 to Switch-2 ensures that if Switch-1 fails, Switch-2 will possess the next lowest priority value in the network, allowing it to seamlessly take over as the backup Root Bridge.

Step-by-Step Solution

1
Identify the STP Root Bridge election criteria.
STP elects the switch with the lowest Bridge ID (Bridge Priority + MAC Address) as the Root Bridge.
Lower numerical values have higher precedence in STP elections.
2
Determine the required priority value for the primary Root Bridge (Switch-1).
Switch-1 priority must be lower than the default value of 3276832768. Setting it to 40964096 guarantees primary root bridge status.
A lower priority value ensures Switch-1 beats all default switches in election comparisons.
3
Determine the required priority value for the secondary backup Root Bridge (Switch-2).
Switch-2 priority must be higher than Switch-1 (40964096) but lower than all other network switches (3276832768). Setting it to 81928192 achieves backup placement.
If Switch-1 fails, Switch-2 will have the next lowest priority (81928192) among remaining operational switches.

Key Concept

STP Root Bridge Election and Priority Mechanics
Estimated Time:1m 15s
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Topics

Spanning Tree Protocol Operations

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