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Zorluk: ZorSpanning Tree Protocol Operations

In a layer 2 switched network running IEEE 802.1D Spanning Tree Protocol (STP), Switch-A is elected as the Root Bridge. Switch-D is evaluating its path options to reach Switch-A and receives BPDUs on two separate interfaces:

- Interface GigabitEthernet0/1 connects to Switch-B (Bridge ID 32768:0000.0000.000B32768\text{:0000.0000.000B}). Switch-B advertises a Root Path Cost of 44. The link speed between Switch-D and Switch-B is 1 Gbps1\text{ Gbps} (path cost 44).
- Interface GigabitEthernet0/2 connects to Switch-C (Bridge ID 32768:0000.0000.000A32768\text{:0000.0000.000A}). Switch-C advertises a Root Path Cost of 44. The link speed between Switch-D and Switch-C is 1 Gbps1\text{ Gbps} (path cost 44).

Which interface on Switch-D will be selected as the Root Port, and what is the primary reason for this selection?

  1. GigabitEthernet0/2, because both paths yield an identical total Root Path Cost of 8, and Switch-C presents a lower Designated Bridge ID than Switch-B.Cevap
  2. B
    GigabitEthernet0/1, because Switch-B has a higher MAC address numerical value, which gives it higher election precedence when path costs are equal.
  3. C
    GigabitEthernet0/1, because traffic over untagged native VLAN trunks is automatically directed to the switch with the higher MAC address to resolve forwarding conflicts.
  4. D
    Both GigabitEthernet0/1 and GigabitEthernet0/2, because STP automatically places ports with equal cumulative path costs into the Forwarding state for Layer 2 load balancing.

Cevap

GigabitEthernet0/2 will be selected as the Root Port because both paths result in an equal total Root Path Cost of 8, and Switch-C provides a lower Designated Bridge ID (ending in 000A) compared to Switch-B (ending in 000B).
When a switch calculates paths to the Root Bridge, it evaluates decision criteria in strict order: 1) Lowest cumulative Root Path Cost, 2) Lowest upstream Designated Bridge ID, 3) Lowest upstream Designated Port ID, and 4) Lowest local Port ID. In this scenario, both paths yield an identical total path cost of 8. The tie is broken by comparing the Designated Bridge IDs of Switch-B and Switch-C. Switch-C has a lower MAC address (0000.0000.000A) than Switch-B (0000.0000.000B), making GigabitEthernet0/2 the Root Port.

Adım Adım Çözüm

1
Calculate the cumulative Root Path Cost for each receiving interface on Switch-D.
Path via Switch-B = 4 (advertised by Switch-B) + 4 (Gigabit link cost) = 8. Path via Switch-C = 4 (advertised by Switch-C) + 4 (Gigabit link cost) = 8.
STP first compares total root path costs to identify the shortest path to the Root Bridge.
2
Evaluate the STP decision criteria tiebreaker since cumulative path costs are equal.
Compare upstream Designated Bridge IDs: Switch-B (32768:0000.0000.000B32768\text{:0000.0000.000B}) vs Switch-C (32768:0000.0000.000A32768\text{:0000.0000.000A}).
When cumulative root path costs are equal, the tiebreaker is the lowest upstream Designated Bridge ID.
3
Determine the winning interface.
Switch-C has the lower MAC address (0000.0000.000A<0000.0000.000B0000.0000.000A < 0000.0000.000B), so interface GigabitEthernet0/2 is selected as the Root Port.
Lower numerical Bridge ID values have higher priority in STP calculations.

Anahtar Kavram

STP Root Port Selection Criteria and Tiebreakers
Tahmini Süre:1m 30s
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Spanning Tree Protocol Operations

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