Link Aggregation with Eth-Trunk

What it is

Link aggregation bundles several physical Ethernet links into one logical link, called an Eth-Trunk on Huawei devices. It increases bandwidth, adds redundancy because traffic moves to the remaining links if one fails, and shares load across the members.

STP sees the Eth-Trunk as one link, so no member is blocked. Huawei supports manual mode, where you choose the members and all of them forward, and LACP mode, where the two ends negotiate with LACP (IEEE 802.3ad) and can keep backup links.

How it works

Basic concepts

Member interfaces must match on settings such as rate and duplex, and you configure Layer 2 settings such as the link type on the Eth-Trunk, not on the members. The maximum number of members depends on the device model.

Load balancing

Traffic is shared per flow: frames of one flow, chosen by fields such as source and destination MAC or IP addresses, always use the same member. This keeps frames in order. Per-packet sharing spreads load more evenly but can reorder frames, so per-flow is the usual choice. The mode is set with load-balance in the Eth-Trunk view.

Manual mode

You create the Eth-Trunk and add members by hand. There is no negotiation and every member is active.

The weakness is fault detection. A manual Eth-Trunk only notices a member going physically down. If a fiber breaks in one direction or a peer is cabled wrong, the link may stay up and drop traffic.

LACP mode

The ends exchange LACPDUs to agree on which members are active. Each end has a system priority and each port has an interface priority; both default to 32768, and a smaller value is better.

1. Actor election. The end with the smaller system priority becomes the Actor. If they tie, the smaller system MAC address wins.
2. Active interface selection. The Actor picks the active interfaces by interface priority, then by interface number, both smaller first, up to the upper threshold.
3. The other end follows. The partner makes the matching interfaces active; the rest are inactive backups.

If an active link fails, a backup becomes active. With preemption (lacp preempt enable), a recovered higher-priority interface takes back its active role after the preemption delay, 30 seconds by default.

Manual vs LACP

Configuration examples

Manual mode, members added in the Eth-Trunk view:

[SW1] interface Eth-Trunk1
[SW1-Eth-Trunk1] mode manual load-balance
[SW1-Eth-Trunk1] trunkport GigabitEthernet 0/0/1 to 0/0/3
[SW1-Eth-Trunk1] port link-type trunk
[SW1-Eth-Trunk1] port trunk allow-pass vlan 10 20

LACP mode with three links, two active and one backup, with SW1 as Actor:

[SW1] lacp priority 100
[SW1] interface Eth-Trunk1
[SW1-Eth-Trunk1] mode lacp
[SW1-Eth-Trunk1] max active-linknumber 2
[SW1-Eth-Trunk1] lacp preempt enable
[SW1-Eth-Trunk1] quit
[SW1] interface GigabitEthernet0/0/1
[SW1-GigabitEthernet0/0/1] eth-trunk 1
[SW1-GigabitEthernet0/0/1] lacp priority 100

You can also add a member from the interface view with eth-trunk 1. Some older VRP versions use mode lacp-static instead of mode lacp. display eth-trunk 1 shows the mode, the Actor and Partner values, and which ports are selected.

A worked example

SW1 and SW2 are joined by GE0/0/1 to GE0/0/3 in LACP mode. SW1 has system priority 100, SW2 keeps 32768, so SW1 is the Actor. With max active-linknumber 2, SW1 makes GE0/0/1 and GE0/0/2 active because they have the lowest interface priority and numbers.

GE0/0/3 is a backup. If GE0/0/1 fails, GE0/0/3 becomes active. When GE0/0/1 recovers, it returns to active only if preemption is enabled, after the 30-second delay.

How the exam asks about this

• "In LACP mode, which device decides the active interfaces?" → The Actor, the end with the smaller system priority, then the smaller MAC.
• "What are the default LACP system priority and interface priority on a Huawei switch?" → Both 32768.
• "Which of the following statements about manual mode is true?" → All member links forward traffic and no LACPDUs are exchanged.