Boost Network Bandwidth & Reliability with Link Aggregation: Concepts & Config

Basic Concept of Link Aggregation

As network scale expands, users demand higher bandwidth and reliability for backbone links. Traditional methods replace high‑speed interface cards or devices, which is costly and inflexible. Link aggregation bundles multiple physical interfaces into a single logical interface, increasing bandwidth without hardware upgrades and improving reliability through backup link mechanisms.

Application Scenarios

In enterprise networks, traffic from all devices converges at the core layer before being forwarded elsewhere. Congestion can occur at this layer. Deploying link aggregation at the core enhances overall data throughput and alleviates congestion, for example, between two switches SWA and SWB connected by multiple member links.

Link Aggregation Explained

Link aggregation combines several physical links between two devices into one logical link.

The logical link’s bandwidth equals the sum of its member links, effectively increasing capacity.

If a member link fails, traffic is switched to another active member, enhancing reliability.

Load balancing distributes traffic across member links, minimizing congestion.

Link Aggregation Modes

Manual Load‑Balancing Mode

Member interfaces are added manually without a control protocol. All active links participate in forwarding, sharing traffic evenly. If a link fails, traffic is redistributed among remaining links. Suitable when devices do not support LACP.

Devices such as ARG3 routers and X7 switches can balance based on MAC addresses, IP addresses, or combinations thereof.

Static LACP Mode

Both ends exchange LACP packets to negotiate aggregation parameters, then designate active and standby interfaces. An Eth‑Trunk is created manually, and member ports are added. Active links carry traffic; standby links serve as backups, switching to active if a failure occurs.

Data Flow Control

All member interfaces must share identical parameters: number, speed, duplex, and flow‑control mode.

Frames within a flow must preserve order; otherwise, out‑of‑order delivery can occur when frames traverse different physical links.

Etn‑Trunk uses per‑flow load balancing based on a hash of MAC/IP addresses, ensuring a flow stays on a single physical link while distributing different flows across links.

Basic Configuration

Layer‑2 Configuration

Example commands create an Eth‑Trunk and add member interfaces. The command interface Eth‑trunk creates the trunk, and trunk_ia identifies it (0‑63). Rules include:

Only delete a trunk with no member ports.

Layer‑2 trunks require layer‑2 member ports; layer‑3 trunks require layer‑3 ports.

Maximum of 8 member ports per trunk.

Member ports must be hybrid interfaces.

A trunk cannot be a member of another trunk.

An Ethernet interface can belong to only one trunk.

All member ports must be of the same type (e.g., all Gigabit).

Ports on different line cards can join the same trunk.

Different speeds may cause congestion on slower ports.

After adding ports, the trunk learns MAC addresses; member ports no longer do.

Viewing Link Aggregation Information

Run display interface eth‑trunk <trunk‑id> to verify successful aggregation and collect traffic statistics. An UP state indicates normal operation; DOWN or Administratively Down signals faults.

Layer‑3 Link Aggregation Configuration

To configure a layer‑3 trunk, create the Eth‑Trunk, then execute undo portswitch to convert it to a layer‑3 interface, after which an IP address can be assigned.

Link Aggregation Protocols

Protocols establish and maintain aggregation negotiations. Dynamic aggregation uses protocols; static aggregation does not. PAGP is Cisco‑specific; LACP follows the IEEE 802.3ad standard.

LACP Use Cases

LACP dynamically aggregates links, expanding bandwidth proportionally and providing automatic backup among member ports.

LACP Negotiation Mode

Dynamic ports default to a management key of zero; static ports use the aggregation group ID as the key.

Viewing LACP Configuration

Use the same display interface eth‑trunk command to inspect LACP settings and interface status.