Concepts

In Figure 3-1, DeviceA and DeviceB are connected through three Ethernet physical links. These three Ethernet physical links are bundled into an Eth-Trunk link, increasing bandwidth and reliability.

Figure 3-1  Eth-Trunk networking

Link aggregation concepts are described as follows:

• LAG and LAG interface

  A link aggregation group (LAG) is a logical link composed of multiple Ethernet links.

  Each LAG corresponds to a logical interface, either a link aggregation interface or an Eth-Trunk. The Eth-Trunk can be used as a common Ethernet interface with one difference: The Eth-Trunk uses one or more member interfaces to forward data.

• Member interface and member link

  The interfaces that constitute an Eth-Trunk are member interfaces. A link corresponding to a member interface is a member link.

• Active and inactive interfaces and links

  There are two types of interfaces in an LAG: active interfaces that forward data and inactive interfaces that do not forward data.

  The link connected to an active interface is an active link, whereas the link connected to an inactive interface is an inactive link.

• Upper threshold for the number of active interfaces

  When the number of active interfaces reaches this threshold, any additional member links will be set to Down. This guarantees higher network reliability by allowing those links to act as backups.

  The upper threshold for the number of active interfaces does not apply to the manual load balancing mode.

• Lower threshold for the number of active interfaces

  When the number of active interfaces falls below the lower threshold, the Eth-Trunk goes Down. This ensures that an active Eth-Trunk has the minimum required bandwidth.

  For example, if the Eth-Trunk is required to provide a minimum bandwidth of 2 Gbit/s and each member link's bandwidth is 1 Gbit/s, the minimum number of Up member links must be set to 2 or larger.

• Link aggregation mode

  There are two link aggregation modes: manual and Link Aggregation Control Protocol (LACP). Table 3-1 compares the two modes.

  Table 3-1  Comparisons between link aggregation modes

  Item	Manual Mode	LACP Mode
  Definition	You must manually create an Eth-Trunk and add member interfaces to the Eth-Trunk. This mode does not require devices to support LACP.	An Eth-Trunk is created using LACP. LACP provides a standard negotiation mechanism for a switching device so that the switching device can be configured to automatically form and start aggregated links. After an aggregated link is formed, LACP is responsible for maintaining the link. If link aggregation conditions or requirements change, LACP can adjust or remove the aggregated link.
  LACP required	No	Yes
  Data forwarding	Generally, all links are active links. If one active link fails, traffic is load balanced among the remaining active links.	Generally, only some links are active links. If an active link fails, the system selects a link among inactive links to replace it. This ensures that the total number of links performing data forwarding remains unchanged.
  Support for inter-device link aggregation	No	Yes
  Fault detection	This mode can only detect member link disconnections, but cannot detect other faults such as link layer faults and incorrect link connections.	This mode can detect member link disconnections and other faults such as link layer faults and incorrect link connections.

  For more information, see Link Aggregation in Manual Mode and Link Aggregation in LACP Mode.

• Link aggregation modes supported by the device

  • Intra-card: Member interfaces of an Eth-Trunk are located on the same card.
  • Inter-card: Member interfaces of an Eth-Trunk are located on different cards.
  • Inter-chassis: Member interfaces of an Eth-Trunk are located on member devices of a CSS. For details, see Link Aggregation in CSS Scenarios.
  • Inter-device: The inter-device link aggregation refers to Enhanced Trunk (E-Trunk). E-Trunk allows links between multiple devices to be aggregated using LACP. For details, see E-Trunk.