Read VLANs and trunks.
Layer 2 loops and spanning tree operation
Ethernet frames do not have an IP-style hop limit to stop a Layer 2 loop. Redundant switching paths can keep frames circulating. Spanning Tree Protocol (STP) creates a loop-free forwarding topology while keeping alternative paths available.
Rapid PVST+ runs a rapid spanning-tree instance per VLAN. Switches select a root bridge using the lowest bridge ID, including priority and MAC-based information. Root ports lead toward the root; designated ports forward for a segment. Alternate paths can be kept in a discarding state. The rapid states are discarding, learning and forwarding.
PortFast and BPDU Guard on edge ports
PortFast lets an edge port reach forwarding quickly. BPDU guard can disable an edge port if it receives an STP message (BPDU). Root guard protects where a superior root should not appear. Loop guard helps prevent certain failures involving missing BPDUs. BPDU filtering changes BPDU handling and can hide loops if used carelessly; it is not interchangeable with BPDU guard.
EtherChannel combines compatible links into one logical port-channel. LACP negotiates aggregation; active/active or active/passive can form a channel, while passive/passive does not initiate one. Members need compatible settings. Distribution usually uses a hash, so one flow does not necessarily use the combined bandwidth of every link.
Verification commands
Use these viewing commands to inspect the idea in this lesson. Start in the environment shown above each example.
show spanning-tree
show etherchannel summaryCommand reference · enter in the stated environment; output varies by device.
Practical exercise
For a two-link LACP bundle, predict the result when both switches are passive. Then explain how the logical link differs from two independent parallel trunks.
Show the answer and explanation
Both passive ends wait instead of initiating LACP. A formed port-channel is treated as one logical STP link; independent trunks are separate paths that STP must evaluate.
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