A network administrator is manually configuring a Cisco switch to become the root bridge f… — Cisco CCNA

A network administrator is manually configuring a Cisco switch to become the root bridge for VLAN 10 using Rapid PVST+. Which value is valid to specify with the command `spanning-tree vlan 10 priority <value>`?

24576Answer
B
24586
C
25000
D
Enabling PortFast on the trunk link to lower priority

Answer

The valid bridge priority setting is 24576, as Rapid PVST+ requires configurable priority values to be in increments of 4096.

In Rapid PVST+, the configurable bridge priority must be a multiple of 4096 (

0, 4096, 8192, \dots, 61440

). The value 24576 is

6 \times 4096

, making it a valid configuration parameter. The switch automatically adds the VLAN ID (10) to the base priority to create the combined Bridge Identifier priority of 24586.

Step-by-Step Solution

Identify the Rapid PVST+ bridge priority structure requirement

Rapid PVST+ uses the 802.1t System ID Extension, dividing the 16-bit Bridge Priority field into a 4-bit priority component and a 12-bit VLAN ID component.

Because 4 bits are reserved for priority, values can only change by steps of

2^{12} = 4096

Evaluate the candidate values for multiples of 4096

24576 / 4096 = 6

, which is an exact integer multiple.

The CLI requires entering the base priority multiplier (

6 \times 4096 = 24576

), and the switch will internally add VLAN 10 to yield a total priority of 24586.

Key Concept

Rapid PVST+ Bridge Priority Increment Calculation

Estimated Time:45s

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