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Configuration Guide - Virtualization

CloudEngine 12800 and 12800E V200R002C50

This document describes the configurations of virtualization, including stack and VS.

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Stack Split and Dual-Active Detection

Stack Split and Dual-Active Detection

Stack Split

After a stack is set up, the master and standby switches periodically send heartbeat packets to maintain the stack state. If communication between the two switches is interrupted due to failures of stack cables or MPUs or power failure or restart of a switch, the stack splits into two standalone switches, as shown in Figure 1-14.

Figure 1-14 Stack split

After a stack splits, the two switches use the same global configuration if they are running normally. In this case, the two switches use the same IP address and MAC address to communicate with other network devices. The address conflict causes a communication failure on the entire network. Dual-active detection (DAD) can be configured to ensure that only one master switch exists after the stack splits.

Dual-Active Detection

Dual-active detection (DAD) is a protocol that can detect stack split and dual-active situations and take recovery actions to minimize impact of a stack split on services.

DAD Detection Modes

DAD can be implemented in the following modes:
  • Direct mode through service ports

    In this mode, DAD is performed through dedicated direct links between member switches, as shown in Figure 1-15.
    Figure 1-15 DAD in direct mode through service ports
    The direct detection links can also be connected through an intermediate device, as shown in Figure 1-16. In direct mode, DAD packets are bridge protocol data units (BPDUs), so the intermediate device must be configured to transparently transmit BPDUs. For details on the configuration method, see Configuring Interface-based Layer 2 Protocol Transparent Transmission in the CloudEngine 12800 and 12800E Series Switches Configuration - Ethernet Switching Configuration Guide.
    Figure 1-16 DAD through direct links to an intermediate device
  • Relay mode through Eth-Trunk interfaces

    In this mode, DAD detection is performed through an inter-device Eth-Trunk link connected to a relay agent, as shown in Figure 1-17. The DAD proxy function must be enabled on the relay agent. Compared with the direct mode, the relay mode does not require additional interfaces because the Eth-Trunk interface can perform DAD relay detection while running other services.

    To enable DAD packets to be forwarded over Eth-Trunk member links, use a switch that supports the DAD proxy function as the relay agent. All Huawei CloudEngine series switches support the DAD proxy function. Huawei S series switches support this function since V200R002C00.

    Figure 1-17 DAD in relay mode through Eth-Trunk interfaces
    The relay agent can be a standalone switch or a stack. That is, two stacks can function as a proxy for each other, as shown in Figure 1-18.
    Figure 1-18 Two stacks as DAD relay agents of each other

    To avoid interference to DAD in the two stacks, configure different domain IDs for the two stacks. In addition, the Eth-Trunk interface used for DAD detection must be different from the Eth-Trunk interface working as the proxy.

  • DAD through management interfaces

    In this mode, links established on management interfaces of the stack member switches are used as DAD links, as shown in Figure 1-19. This mode can be used when all stack member switches connect to the management network through their management interfaces. This mode does not occupy additional ports and does not require a DAD relay agent.

    To implement DAD through management interfaces, ensure that IP addresses are configured for management interfaces. After member switches set up a stack, only one management interface MEth0/0/0/0 is displayed for the stack. You only need to configure an IP address for this management interface.

    Figure 1-19 DAD through management interfaces
    As shown in Figure 1-20, when no management network exists, DAD can be implemented when stack member switches directly connect to each other through management interfaces. In this situation, the management interfaces must also have IP addresses configured.
    Figure 1-20 DAD through directly connected management interfaces
  • DAD through stack member ports

    In this mode, links established between stack member ports of the stack member switches are used as DAD links, as shown in Figure 1-21. This mode uses stack links as DAD links and do not occupy additional ports.

    DAD through stack member ports can be used only when the stack is set up through MPU connection.

    Figure 1-21 DAD through stack member ports

Dual-Active Conflict Handling and Fault Recovery

After DAD is configured in a stack, the master switch periodically sends DAD competition packets over the detection links. After the stack splits, the switches exchange DAD competition packets and compare information in the received DAD competition packet with local information. If local information is better, the local switch remains in Active state and continues forwarding service packets. If the received information is better, the switch stack turns to the Recovery state. In this case, all the service interfaces except the excluded ones on the switch are Error-Down and stop forwarding service packets.

After a stack splits into two stacks, the two stacks compare the following items in the listed order to determine the Active/Recovery state (the election ends when a winning stack is found):
  1. Stack priority: The stack to which the switch with the highest stack priority belongs wins.

  2. MAC address of switches: The stack to which the switch with the smallest MAC address belongs wins.


A dual-active conflict will not be detected among the switches that do not support stack setup.

After the stack links recover, the two stacks merge into one. The switches in Recovery state restart and restore the Error-Down service interfaces. Then the entire stack recovers.

If the switch in Active state also fails before the faulty stack links recover, remove this switch from the network first, and then use a command to start the switches in Recovery state, enabling the switches to take over services on the original switch in Active state. After the faulty switch and stack links recover, connect the switch to the network again so that the stacks can merge.

Updated: 2019-03-21

Document ID: EDOC1000166610

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