A network administrator configures a LACP-based link aggregation group (LAG) consisting of four 1Gbps physical links between two switches. After deployment, a single large file transfer between two hosts is measured at only 1Gbps of throughput rather than the expected aggregate bandwidth. Which of the following BEST explains this behavior?
- AThe switches must be rebooted to enable full 4Gbps throughput for a single flow
- BLACP failed to negotiate and only one link is active in the bundle
- CA single conversation between two hosts is typically hashed to only one physical link based on the load-balancing algorithm
- DLACP limits any single LAG to a maximum of 1Gbps regardless of the number of member links
Show answer & explanationAnswer & explanation
Correct answer: C. A single conversation between two hosts is typically hashed to only one physical link based on the load-balancing algorithm
Link aggregation increases aggregate bandwidth across multiple simultaneous flows, but a single conversation (e.g., one file transfer's flow, typically hashed by source/destination MAC or IP) is generally mapped to only one physical link by the load-balancing hash algorithm. This is expected behavior, not a failure, and is why LAGs benefit multiple simultaneous streams rather than a single large transfer.
Why the other options are wrong
- A. Rebooting switches has no effect on the fundamental per-flow hashing behavior of LAGs.
- B. If negotiation failed, the LAG would likely not form correctly system-wide, not just limit a single flow.
- D. LACP does not impose an artificial 1Gbps ceiling; the limitation is due to per-flow hashing.
LAG Load Balancing Per-Flow
Link aggregation groups distribute traffic across member links based on a hash of parameters like source/destination MAC or IP, meaning a single conversation is usually confined to one physical link.
- Aggregate bandwidth benefits multiple flows, not a single flow
- Hash algorithms use MAC/IP/port combinations
- A single large transfer will not exceed one link's speed
Memory trick: One conversation, one lane — the highway only helps with many cars.