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Zorluk: OrtaSpanning Tree Protocol Operations

Three enterprise switches (SW1, SW2, and SW3) are connected in a triangular loop running standard IEEE 802.1D Spanning Tree Protocol (STP). All three switches retain the default bridge priority of 32768, and their MAC addresses are as follows:
- SW1: 00:1A:2B:3C:4D:01
- SW2: 00:1A:2B:3C:4D:02
- SW3: 00:1A:2B:3C:4D:03

The physical links and their associated STP path costs are:
- Link between SW3 and SW1: 100 Mbps (STP path cost = 19)
- Link between SW3 and SW2: 1 Gbps (STP path cost = 4)
- Link between SW2 and SW1: 1 Gbps (STP path cost = 4)

Which switch port on SW3 will be elected as the Root Port, and what is its calculated Root Path Cost?

  1. The port connected to SW2, with a Root Path Cost of 8Cevap
  2. B
    The port connected directly to SW1, with a Root Path Cost of 19
  3. C
    The port connected to SW2, with a Root Path Cost of 4
  4. D
    The port connected directly to SW1, with a Root Path Cost of 8

Cevap

The port connected to SW2, with a Root Path Cost of 8
Because all switches share the default priority of 32768, SW1 is elected as the Root Bridge based on having the lowest MAC address (00:1A:2B:3C:4D:01). SW3 must select a single Root Port that offers the lowest cumulative cost to reach SW1. The direct link from SW3 to SW1 operates at 100 Mbps with a cost of 19. The indirect path from SW3 through SW2 to SW1 uses two 1 Gbps links, resulting in a total path cost of 4 + 4 = 8. Since 8 is less than 19, SW3 elects the port facing SW2 as its Root Port with a Root Path Cost of 8.

Adım Adım Çözüm

1
Determine the Root Bridge by comparing Switch Priority and MAC Address.
All switches have equal priority (32768). SW1 has the lowest MAC address (00:1A:2B:3C:4D:01) and is elected as the Root Bridge.
STP selects the switch with the lowest Bridge ID (Priority + MAC) as the Root Bridge.
2
Calculate the cumulative Root Path Cost for all possible paths from SW3 to the Root Bridge (SW1).
Direct path (SW3 → SW1): Cost = 19. Indirect path (SW3 → SW2 → SW1): Cost = 4 + 4 = 8.
Root Path Cost is the sum of all ingress port path costs along the path toward the Root Bridge.
3
Select the Root Port on SW3 based on the lowest cumulative Root Path Cost.
The port connected to SW2 has a lower total cost (8 < 19) and is elected as SW3's Root Port.
Every non-root switch elects exactly one Root Port corresponding to its shortest path to the Root Bridge.

Anahtar Kavram

STP Root Bridge election and cumulative Root Path Cost calculation for Root Port selection.
Tahmini Süre:1m 30s
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Spanning Tree Protocol Operations

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