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Fat AP and Cloud AP V200R008C00 CLI-based Configuration Guide

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This section describes principles of link aggregation.


As shown in Figure 7-15, DeviceA and DeviceB are connected through three Ethernet physical links. These three Ethernet physical links are bundled into an Eth-Trunk link. The bandwidth of the Eth-Trunk link is the sum of bandwidth of the three Ethernet physical links, so bandwidth is increased. The three Ethernet physical links back up each other, which improves reliability.

Figure 7-15  Eth-Trunk networking

The link aggregation interface can be used as a common Ethernet interface to implement routing protocols and other services. Unlike a common Ethernet interface, the link aggregation interface uses one or more member interfaces to forward data.

Link aggregation concepts are described as follows:

  • Link aggregation, LAG (Link Aggregation Group), and link aggregation interface

    Link aggregation is a method of bundling a group of physical interfaces into a logical interface to increase bandwidth and improve reliability.

    An LAG is the logical link bundled by many Ethernet links.

    Each LAG corresponds to a logical interface, that is, link aggregation interface or Eth-Trunk.

  • Member interface and member link

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

  • Active and inactive interfaces and links

    Member interfaces can be classified into active interfaces, which forward data and inactive interfaces, which do not.

    Links connected to active interfaces are called active links, and links connected to inactive interfaces are called inactive links.

  • Upper threshold for the number of active interfaces

    When the number of active interfaces reaches this threshold, the bandwidth of the Eth-Trunk will not increase even if more member links go Up. This guarantees higher network reliability. When the number of active member interfaces reaches the upper threshold, additional active member interfaces are set to Down.

    For example, 8 trouble-free member links are bundled into a trunk link, each with a bandwidth of 1 Gbit/s. The trunk link, however, only needs to provide a maximum bandwidth of 5 Gbit/s. By setting the maximum number of Up member links to 5 or a greater, any unselected Up links automatically enter backup status, improving reliability.


    The upper threshold for the number of active interfaces is inapplicable to the manual load balancing mode. For details about the manual load balancing mode, see Link Aggregation in Manual Load Balancing Mode.

  • Lower threshold for the number of active interfaces

    When the number of active interfaces falls below this threshold, the trunk interface goes Down. This guarantees the trunk interface a minimum available bandwidth.

    For example, if the trunk interface 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 a greater.

Forwarding Principle

As shown in Figure 7-16, an Eth-Trunk link is deployed on the data link layer, that is, between the physical layer and the MAC sub-layer.

Figure 7-16  Eth-Trunk interface in the Ethernet protocol stack

An Eth-Trunk interface is assumed to be a physical interface on the MAC sub-layer. Therefore, frames transmitted in the MAC sub-layer only need to be delivered to the Eth-Trunk module that maintains an Eth-Trunk forwarding table.

The Eth-Trunk forwarding table is composed of the following parts:

  • HASH-KEY value

    The key value is calculated through the hash algorithm on the MAC address or IP address in the packet.

  • Interface number

    Eth-Trunk forwarding entries are relevant to the number of member interfaces in the Eth-Trunk. Different HASH-KEY values are mapped to different outbound interfaces.

    For example, an Eth-Trunk supports a maximum of eight member interfaces. If four physical interfaces, 1, 2, 3, and 4, are bundled into an Eth-Trunk, the Eth-Trunk forwarding table contains four entries, as shown in Figure 7-17. In the Eth-Trunk forwarding table, the HASH-KEY values are 0, 1, 2, 3, 4, 5, 6, and 7, and the corresponding interface numbers are 1, 2, 3, 4, 1, 2, 3, and 4.

Figure 7-17  Example of an Eth-Trunk forwarding table

The Eth-Trunk module forwards a frame according to the Eth-Trunk forwarding table. The forwarding process is as follows:

  1. The Eth-Trunk module receives a frame from the MAC sub-layer, and then extracts its source MAC address/IP address or destination MAC address/IP address.

  2. The Eth-Trunk module calculates the HASH-KEY value using the hash algorithm.

  3. Based on the HASH-KEY value, the Eth-Trunk module searches the Eth-Trunk forwarding table for the interface number, and then sends the frame from the corresponding interface.

Link Aggregation in Manual Load Balancing Mode

In manual load balancing mode, you need to manually create an Eth-Trunk interface and add member interfaces to the Eth-Trunk interface, without the assistance of the LACP protocol. In this mode, all active links load balance the traffic evenly. If an active link fails, the other active links share the traffic evenly. If a high link bandwidth between two directly connected devices is required but the peer device does not support the LACP protocol, you can use the manual load balancing mode.

Load Balancing Using Link Aggregation

A data flow is a group of data packets with one or more identical attributes. The attributes refer to the source MAC address, destination MAC address, source IP address, destination IP address, source TCP/UDP port number, and destination TCP/UDP port number.

Because there are multiple physical links between devices of the Eth-Trunk, the first data frame of the same data flow is transmitted on one physical link, and the second data frame may be transmitted on another physical link. In this case, the second data frame may arrive at the peer device earlier than the first data frame. As a result, packet mis-sequencing occurs.

To prevent packet mis-sequencing, Eth-Trunk uses the load balancing mechanism. This mechanism uses the hash algorithm to calculate the address in a data frame and generates a hash key value. Then the system searches for the outbound interface in the Eth-Trunk forwarding table based on the generated hash key value. Each MAC or IP address corresponds to a hash key value, so the system uses different outbound interfaces to forward data. This mechanism ensures that frames of the same data flow are forwarded on the same physical link and implements flow-based load balancing. Flow-based load balancing ensures the sequence of data transmission, but reduces the bandwidth usage.

You can use the following load balancing modes based on actual networking:

  • Based on source MAC addresses of packets
  • Based on destination MAC addresses of packets
  • Based on source IP addresses of packets
  • Based on destination IP addresses of packets
  • Based on the Exclusive-Or result of source and destination MAC addresses of packets
  • Based on the Exclusive-Or result of source and destination IP addresses of packets
Updated: 2019-01-11

Document ID: EDOC1000176006

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