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[DCN] Understanding three inter-device link aggregation technologies: stacking, M-LAG, and M-LAG Lite

Latest reply: May 17, 2019 09:23:20 293 1 3 0

For link aggregation between standalone

devices, if the Eth-Trunk or the remote device fails, the switch or server
cannot communicate with the remote device. This problem can be resolved by
deploying inter-device link aggregation, which allows a switch or server to be
dual-homed to two devices, thereby achieving device-level link reliability.

Based on the networking of uplink devices,

CloudEngine series switches support three inter-device link aggregation
technologies: stacking, M-LAG, and M-LAG Lite.

1. Stacking

Interfaces on member switches in a stack

can be bundled into an Eth-Trunk.

This inter-device link aggregation mode is

applied to scenarios where the stack is connected to other devices, and
protects the link between upstream and downstream devices. The Eth-Trunk can
still work even if a member switch fails or one link of the Eth-Trunk fails,
ensuring reliable transmission of data traffic. This prevents single-point
failures of a member device in a stack and greatly improves the network-wide

For details, see Stack Configuration in the

CloudEngine 12800 and 12800E Series Switches
Configuration Guide - Virtualization Configuration Guide.

Physical and logical topologies of inter-device link aggregation in a stack


2. M-LAG

M-LAG allows two access switches in the

same state to perform link aggregation negotiation with the access device. InFigure
1-2, the
access device communicates with the M-LAG through link aggregation, achieving
device-level reliability instead of only card-level reliability. The M-LAG is a
dual-active system that is composed of two access switches.

M-LAG is a horizontal virtualization

technology that virtualizes two M-LAG devices into one logical device, that is,
a unified Layer 2 logical node. In practice, M-LAG provides loop-free
networking between aggregation and access layers and replaces STP. Compared
with STP, M-LAG provides clear logical topology and better link use efficiency.

M-LAG master and backup devices forward

data simultaneously and their forwarding behaviors are the same. The forwarding
behaviors of M-LAG master and backup device roles are different only in
scenarios where faults occur.

For details, see M-LAG Configuration in the

CloudEngine 12800 and 12800E Series
Switches Configuration Guide - Ethernet Switching Configuration Guide.

Figure 1-2 : Physical and logical topologies of inter-device link aggregation in an M-LAG system


3. M-LAG Lite

Figure 1-3shows

the inter-device link aggregation on the M-LAG Lite networking. Compared with
inter-device link aggregation in a stack or M-LAG system, inter-device link
aggregation in an M-LAG Lite system removes the heartbeat cable (CSS link or
peer-link) required for establishing a stack/M-LAG between devices.
Inter-device link aggregation in an M-LAG Lite system simplifies deployment,
saves costs, and ensures that services are not interrupted during the upgrade.


1-3, the
same Eth-Trunk ID, LACP system ID, LACP system priority, and different numbers
of Eth-Trunk member interfaces are configured on DeviceB and DeviceC. (On one
member device, thelacp
port-id-extension enablecommand is run in views of all the member
interfaces to increase the number of each member interface by 32768.) In this
way, the inter-device Eth-Trunk interface negotiation can be successful. The
two devices evenly load balance data. When one device fails, traffic can be
forwarded through the other device, implementing device-level reliability.

DeviceB and DeviceC must be Layer 3

gateways and cannot be Layer 2 transparent transmission devices. If an upstream
device functions as the gateway, the ARP entries of the server or switch
learned by the gateway have two egresses, causing MAC address flapping.
Therefore, the devices to which the server or switch connects in M-LAG Lite
mode must be Layer 3 gateways.

For details about M-LAG Lite, see

"Example for Configuring M-LAG Lite" in Configuration Examples for
Comprehensive Scenarios in theCloudEngine
12800 and 12800E Series Switches Configuration Guide - Typical Configuration

Figure 1-3 : Inter-device link aggregation in M-LAG Lite networking


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