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Zorluk: OrtaSpanning Tree Protocol Operations

An enterprise network implements Rapid Spanning Tree Protocol (IEEE 802.1w) across its access switches. Switch-A is elected as the Root Bridge. Switch-B is connected directly to Switch-A using two parallel 1 Gbps Ethernet links: Link 1 connects Switch-A interface GigabitEthernet0/1 to Switch-B interface GigabitEthernet0/1, while Link 2 connects Switch-A interface GigabitEthernet0/2 to Switch-B interface GigabitEthernet0/2. All link speeds are identical, and all port priorities remain at default values (128128). Which port role and operational state will Switch-B assign to its GigabitEthernet0/2 interface?

  1. Alternate port in a discarding stateCevap
  2. B
    Backup port in a discarding state
  3. C
    Root port in a forwarding state
  4. D
    Designated port in a forwarding state

Cevap

Alternate port in a discarding state
When a non-root switch receives Root Bridge BPDUs over multiple parallel links of identical cost from the same upstream switch, it breaks the tie by selecting the interface connected to the upstream port with the lowest numerical Port ID (Port Priority + Port Number). Switch-A's GigabitEthernet0/1 has a lower Port ID (128.1128.1) than GigabitEthernet0/2 (128.2128.2), so Switch-B selects its GigabitEthernet0/1 as the single Root Port. Under IEEE 802.1w (RSTP), the remaining redundant port receiving superior root BPDUs from another switch is assigned the Alternate port role and placed in a discarding state.

Adım Adım Çözüm

1
Determine Root Bridge status and path cost for both links.
Switch-A is the Root Bridge. Both links are 1 Gbps with equal cumulative root path cost (44).
Since path costs are tied, the switch must proceed to subsequent tiebreakers.
2
Evaluate the upstream Neighbor Bridge ID tiebreaker.
Both links connect to the exact same upstream switch (Switch-A), resulting in identical upstream Bridge IDs.
When upstream Bridge IDs are tied, the switch evaluates the upstream Port Priority and Port ID.
3
Compare upstream Port IDs from Switch-A.
Switch-A transmits BPDUs with Port ID 128.1128.1 on GigabitEthernet0/1 and Port ID 128.2128.2 on GigabitEthernet0/2. 128.1<128.2128.1 < 128.2.
Lower numerical port ID is preferred. Switch-B selects its GigabitEthernet0/1 (connected to Switch-A Gi0/1) as the Root Port.
4
Determine the role and state for Switch-B's GigabitEthernet0/2 interface.
GigabitEthernet0/2 receives BPDUs from another switch interface providing an extra path to the Root Bridge. In 802.1w (RSTP), this interface becomes an Alternate port in a discarding state.
Alternate ports provide redundant paths toward the root bridge and remain in a discarding state to prevent switching loops.

Anahtar Kavram

RSTP Root Port Election & Alternate Port Role Assignment
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Konular

Spanning Tree Protocol Operations

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