300-320 Question #377: Real Exam Question with Answer & Explanation

The correct answer is A: When redundant Layer 2 links, that are not part of a single EtherChannel or bundle, exist between. Spanning Tree Protocol (STP) exists to prevent Layer 2 (Ethernet) broadcast storms and loops. It is required whenever redundant Layer 2 links exist between switches that are NOT part of an EtherChannel or port-channel bundle. EtherChannel bundles multiple physical links into a si

Question

In what situation must spanning-tree be implemented?

Options

AWhen redundant Layer 2 links, that are not part of a single EtherChannel or bundle, exist between
BWhen redundant Layer 3 links, that are not part of a single EtherChannel or bundle, exist between
CBetween Distribution and Core switches when interfaces are configured with "no switchport"
DBetween Distribution and Core switches when VSS is configured

Explanation

Spanning Tree Protocol (STP) exists to prevent Layer 2 (Ethernet) broadcast storms and loops. It is required whenever redundant Layer 2 links exist between switches that are NOT part of an EtherChannel or port-channel bundle. EtherChannel bundles multiple physical links into a single logical link, so STP sees only one link and no loop is formed. Redundant Layer 3 links (B) do not create Layer 2 loops - routing protocols handle loop prevention at Layer 3. When interfaces are configured with 'no switchport' (C), they operate as routed (Layer 3) ports, eliminating the Layer 2 loop concern. VSS (Virtual Switching System) (D) makes two physical switches appear as a single logical switch, so redundant links between VSS members form a port-channel and no STP loop exists. Therefore, STP is mandatory only for redundant, unbundled Layer 2 links between switches.

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