What is Link Aggregation?
Learn what link aggregation is, how LACP bundles links for bandwidth and failover, with a real config and interview Q&A.
Expected Interview Answer
Link aggregation combines multiple physical network links between two devices into a single logical link, increasing available bandwidth and providing redundancy so the connection survives the failure of any single cable or port.
Standardized as IEEE 802.3ad (and its dynamic control protocol LACP, 802.1AX), link aggregation bundles two or more Ethernet ports — say, four 1 Gbps links — into one logical channel that upper-layer protocols treat as a single interface, though the aggregate bandwidth is realized through traffic distribution across the member links rather than a single flow automatically getting the full combined speed. Traffic is distributed across member links using a hashing algorithm (commonly based on source/destination MAC, IP, or port) so that a single flow typically rides one physical link at a time while different flows fan out across the bundle, spreading load in aggregate. If one physical link in the bundle fails, LACP detects it and traffic continues flowing over the remaining links without interrupting the logical connection, which is the redundancy benefit. Link aggregation is commonly used between switches, and between a switch and a server or storage array that needs both higher throughput and resilience against a single cable or NIC failure.
- Increases aggregate bandwidth across multiple physical links
- Provides automatic failover if one member link goes down
- Presents as a single logical interface to upper layers
- LACP dynamically negotiates and monitors the bundle's health
AI Mentor Explanation
Link aggregation is like a ground crew running four identical drainage pipes side by side out of a waterlogged outfield instead of relying on one pipe — together they clear water faster, and if one pipe clogs, the other three keep draining without stopping the job. Water (traffic) is split across whichever pipes are flowing best at the time, not doubled through a single pipe. This bundling of multiple physical channels into one effective drainage system is exactly what link aggregation does with network cables.
Step-by-Step Explanation
Step 1
Bundle physical links
Two or more Ethernet ports between the same pair of devices are grouped into one logical channel.
Step 2
Negotiate with LACP
LACP (802.1AX) dynamically negotiates which links are active members of the bundle and monitors their health.
Step 3
Hash traffic across links
A hashing algorithm distributes different flows across member links, spreading aggregate load.
Step 4
Failover on link loss
If a member link fails, LACP removes it from the bundle and traffic continues over the remaining links.
What Interviewer Expects
- Clear definition: multiple physical links presented as one logical link
- Knows LACP (802.1AX/802.3ad) as the standard control protocol
- Understands a single flow usually still rides one physical link (hashing, not true speed summing)
- Mentions both bandwidth and redundancy/failover as benefits
Common Mistakes
- Assuming a single file transfer automatically gets the full combined bandwidth
- Confusing link aggregation with load balancing at the application layer
- Not knowing LACP is the dynamic negotiation protocol behind most aggregation setups
- Forgetting both ends of the link (both switches, or switch and server) must support and agree on aggregation
Best Answer (HR Friendly)
“Link aggregation is like bundling several network cables together so they act as one faster, more reliable connection — similar to running several garden hoses side by side instead of one. It boosts overall throughput and means if one cable is accidentally unplugged, the connection keeps working over the others instead of going down.”
Code Example
# Create a bonded interface using 802.3ad (LACP) mode
sudo ip link add bond0 type bond mode 802.3ad
# Add two physical interfaces as members of the bond
sudo ip link set eth0 down
sudo ip link set eth0 master bond0
sudo ip link set eth1 down
sudo ip link set eth1 master bond0
sudo ip link set bond0 up
# Check bond status and active member links
cat /proc/net/bonding/bond0Follow-up Questions
- How does the hashing algorithm decide which member link a flow uses?
- What is the difference between static link aggregation and LACP?
- Why must both ends of an aggregated link agree on the configuration?
- How does link aggregation differ from ECMP (equal-cost multi-path) routing?
MCQ Practice
1. What standard protocol dynamically negotiates link aggregation?
LACP (Link Aggregation Control Protocol, IEEE 802.1AX) dynamically negotiates and monitors aggregated links.
2. Does a single traffic flow typically get the full combined bandwidth of an aggregated link?
Hashing algorithms usually keep a single flow on one physical link; aggregate bandwidth benefits multiple concurrent flows.
3. What happens when one member link in an LACP bundle fails?
LACP detects the failed link and continues forwarding traffic over the remaining healthy member links.
Flash Cards
What is link aggregation? — Combining multiple physical links into one logical link for bandwidth and redundancy.
What standard controls it dynamically? — LACP, defined in IEEE 802.1AX (formerly part of 802.3ad).
Does one flow get full combined speed? — Usually no — hashing keeps a single flow on one member link; multiple flows spread across the bundle.
Key redundancy benefit? — If one member link fails, traffic keeps flowing over the remaining links automatically.