How Link Aggregation Works
When you configure LACP between a NAS and a switch, the two devices negotiate using LACP PDUs (Protocol Data Units) to agree on which ports to include in the aggregated group. Once the LAG (Link Aggregation Group) is established, the switch and NAS treat the bonded ports as a single logical interface. Traffic is distributed across the member links using a hashing algorithm — typically based on source/destination MAC addresses, IP addresses, or TCP/UDP port numbers.
A key point: link aggregation does not give a single TCP connection more than the speed of one physical link. A single file transfer uses one hash value and therefore one physical port. The bandwidth benefit is realized across multiple concurrent connections — for example, four clients simultaneously accessing a NAS each get a full gigabit link rather than sharing one.
Active vs Passive LACP
LACP operates in two modes on each side of the link. Active mode: the interface actively sends LACP PDUs to initiate and maintain the aggregation. Passive mode: the interface responds to LACP PDUs but does not initiate them. For a LAG to form, at least one side must be in active mode. Active-Active always works. Active-Passive works. Passive-Passive does not form a LAG.
Most configurations use active mode on both sides for simplicity. Some switches and NAS devices default to active; check your specific hardware's documentation. Some devices also support static or manual LAG without LACP negotiation — the ports are bonded without any protocol, which works but provides no automatic failover detection if one end misconfigures.
LACP on a NAS: Real-World Bandwidth
The most common homelab LACP use case: a Synology or QNAP NAS with two Ethernet ports connected to two ports on a managed switch via LACP. With a single client accessing the NAS, you get one gigabit — no different from before. With two or more clients simultaneously (or one client running multiple parallel transfers), each flow can use a different physical link, aggregating available bandwidth. The NAS appears as a single IP address with combined bandwidth available across multiple concurrent sessions.
For a homelab with one or two workstations accessing the NAS, LACP provides failover redundancy (one cable can fail and the connection stays up) but limited bandwidth benefit. The bandwidth benefit becomes significant when three or more clients access the NAS simultaneously, or when running a Proxmox cluster where live migration and storage I/O coexist.
Link Aggregation Modes and Protocols
| Mode / Protocol | Negotiation | Failover Detection | Load Balancing | Support Required |
|---|---|---|---|---|
| LACP (802.3ad) | Yes (active/passive) | Yes (automatic) | Hash-based | Both sides must support 802.3ad |
| Static LAG | No | Link-level only | Hash-based | Both sides must match config |
| Linux bonding (mode 4) | Yes (LACP) | Yes | Hash-based | Switch must support LACP |
| Linux bonding (mode 0 - balance-rr) | No | No LACP | Round-robin per packet | Switch may not support; causes out-of-order packets |
| Linux bonding (mode 1 - active-backup) | No | Yes | None (one active link) | Works with any switch; pure failover only |
Frequently Asked Questions
Does LACP double my network speed for a single file transfer?
No. A single TCP connection uses one physical link in the LAG because LACP hash-based load balancing assigns each flow to one link. LACP increases total aggregate throughput across multiple simultaneous connections. For a server with many clients connecting concurrently, LACP provides meaningful bandwidth increase. For a single workstation downloading from a NAS, LACP provides redundancy but no speed increase on that one transfer.
What managed switches support LACP?
Most managed switches support LACP (802.3ad), including consumer-grade options from TP-Link (TL-SG108E and above), Netgear (GS308E, GS316EP), and Ubiquiti UniFi. Unmanaged switches do not support LACP. Check your switch's product specifications for 802.3ad or Link Aggregation Group (LAG) support before purchasing if you plan to use bonding.
How do I configure LACP on a Synology NAS?
In DSM (DiskStation Manager): Control Panel → Network → Network Interface → Create → Create Bond. Select the ports to include, choose the bond mode (IEEE 802.3ad Dynamic Link Aggregation for LACP, or Active Backup for simple failover without a managed switch). The switch port group must also be configured as a LAG with LACP enabled on the same ports. After configuration, the NAS presents a single virtual interface with one IP address.
Can I use LACP to connect a NAS directly to a server?
Direct LACP between two devices (NAS to server, no switch) is possible but less useful — the aggregate link appears as a single connection between those two specific devices. With a switch in between, the aggregate bandwidth is available to all switch ports simultaneously. For a direct NAS-to-server connection, 2.5GbE or 10GbE on a single port is often a better solution than LACP over multiple 1GbE ports.