Questions: Link Aggregation Control Protocol (LACP)

Question 1

A server is connected to a switch via a 4-link LACP bond (4 × 1 Gbps). An administrator starts a single large file transfer from the server to a client. What maximum throughput can this transfer achieve?

• A. 4 Gbps — all four physical links combine their bandwidth for any single transfer
• B. 2 Gbps — LACP automatically splits large flows across two links for load balancing
• C. 1 Gbps — the hashing algorithm maps a single flow to one physical link, and that link's speed is the ceiling
• D. Variable, up to 4 Gbps — LACP dynamically routes individual packets across all links in round-robin fashion

Answer: (C)

LACP's hashing algorithm maps each flow (defined by its source/destination address pair or port numbers) to a specific physical link and keeps all packets of that flow on the same link to preserve ordering. A single TCP connection is a single flow — it maps to exactly one physical link and cannot exceed that link's bandwidth. The 4 Gbps aggregate capacity is realized only when many simultaneous flows from different source-destination pairs spread across all four links. This is the most important practical limitation of link aggregation.

Question 2

A network engineer connects two switches with two parallel cables (no LACP configured) hoping to double bandwidth. What most likely happens?

• A. Bandwidth doubles as the switches automatically distribute traffic across both cables
• B. Spanning Tree Protocol detects a loop and blocks one of the parallel links, restoring single-link bandwidth
• C. Both links remain active in an active-standby configuration: one carries traffic, the other waits for failover
• D. The second link is used only for broadcast traffic, reducing congestion on the primary link

Answer: (B)

Without link aggregation, STP sees two parallel paths between the same two switches as a bridging loop. To prevent broadcast storms, STP blocks all but one path. This is exactly the problem LACP solves: the entire aggregated bundle appears to STP as a single logical link, so STP does not block any member ports. The engineer would need to configure LACP on both switches to gain bandwidth and redundancy without STP blocking.

Question 3

LACP requires both sides of the link to be configured for it; connecting an LACP-configured port to a switch that only supports static LAG will not form a functional aggregation.

• T. True
• F. False

Answer: True

LACP works by exchanging LACPDUs between both sides to dynamically negotiate which ports join the aggregation group. If one side does not speak LACP, the negotiation fails and no LAG is formed. Static LAG (manually configured port channel without LACP) is a separate mechanism that does not require LACP support from the peer, but it lacks LACP's ability to automatically detect misconfigurations, link failures, and incompatible ports.

Question 4

When one physical link in an LACP bundle fails, the logical link (as seen by the rest of the network) goes down until STP reconverges and selects a new path.

• T. True
• F. False

Answer: False

This is the key resilience advantage of LACP over simple parallel links. When one member link fails, LACP detects the loss of LACPDUs within seconds and removes that link from the bundle, redistributing traffic across the surviving member links. The logical LAG interface stays up (at reduced bandwidth). No STP reconvergence is needed because STP still sees one logical link — it has not gone down. This is faster and less disruptive than STP-based failover.

Question 5

Explain why adding a 4-link LACP bond does not guarantee a 4x speedup for a single client downloading a large file from a server.

Model answer: LACP distributes flows across member links using a hash of source and destination addresses (or port numbers). All packets belonging to the same flow — same source IP, destination IP, and port combination — always travel over the same physical link to preserve packet ordering. A single file download is a single TCP flow, so it is hashed to one link and limited to that link's bandwidth. The 4x aggregate bandwidth is only realized when many different flows (from different clients or connections) hash to different links simultaneously.

This surprises many engineers. The mental model of 'four pipes = four times the water' is wrong for individual flows. The correct mental model is 'four separate lanes for four separate cars — one car still only uses one lane.' To saturate a 4-link bond, you need four or more concurrent flows with different address/port hashes. A single client downloading a single file will see at most 1 Gbps no matter how many links are in the bond.