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Configuration Guide - Virtualization

CloudEngine 8800, 7800, 6800, and 5800 V200R005C10

This document describes the configurations of virtualization, including stack and SVF.
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Stack Split and Dual-Active Detection

Stack Split and Dual-Active Detection

Stack Split

If you remove some member switches from a running stack without powering off the switches or if multiple stack cables fail, the stack splits into multiple stacks.

Depending on the locations of the master and standby switches after a split, either of the following situations occurs:
  • The original master and standby switches are in the same stack after the split.

    The original master switch recalculates the stack topology, deletes topology information of the removed member switches, and synchronizes new topology information to the other member switches in the new stack. The removed switches restart, set up a new stack, and elect a new master switch.

    As shown in Figure 1-9, the original master switch (SwitchA) and standby switch (SwitchB) are in the same stack after the split. SwitchA deletes topology information related to SwitchD and SwitchE and synchronizes new topology information to SwitchB and SwitchC. After SwitchD and SwitchE restart, they set up a new stack.

    Figure 1-9 Original master and standby switches in the same stack after a split
  • The original master and standby switches are in different stacks after the split.

    The original master switch specifies a new standby switch in its stack, recalculates the topology, and synchronizes topology information to the other member switches in the stack. The original standby switch becomes the master switch in its stack. It then recalculates the topology, synchronizes topology information to the other member switches in the stack, and specifies a new standby switch.

    As shown in Figure 1-10, the original master switch (SwitchA) and standby switch (SwitchB) are in different stacks after the split. SwitchA specifies SwitchD as the new standby switch, recalculates the stack topology, and synchronizes new topology information to SwitchD and SwitchE. In the other stack, SwitchB becomes the master switch. It then recalculates the topology, synchronizes topology information to SwitchC, and specifies SwitchC as the new standby switch.

    Figure 1-10 Original master and standby switches in different stacks after a split

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 1-11.
    Figure 1-11 DAD in direct mode through service ports
    The direct detection links can also be connected through an intermediate device, as shown in Figure 1-12. 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 Tunneling in the CloudEngine 8800, 7800, 6800, and 5800 Series Switches Configuration - Ethernet Switching Configuration Guide.
    Figure 1-12 DAD through direct links to an intermediate device
    If three or more member switches are stacked and DAD in direct mode through service ports is used, the full-mesh connection mode is recommended between stack members, as shown in Figure 1-13. That is, every two stack members are connected using cables.
    Figure 1-13 Full-mesh connection between stack members

    In full-mesh connection mode, every two stack members need to be connected using cables. When there are a large number of stack members, a large number of service ports will be occupied. Therefore, if there are three or more stack members, you are advised to first use DAD in relay mode through Eth-Trunk interfaces.

    To ensure high reliability, you are advised not to use any other connection mode, including the connection mode shown in Figure 1-14, except the full-mesh connection mode.
    Figure 1-14 Connection mode that is not recommended
  • 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 1-15. The DAD relay 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 relay function as the relay agent. All Huawei CloudEngine series switches support the DAD relay function. Huawei S series switches support this function since V200R002C00.

    Figure 1-15 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 relay agent for each other, as shown in Figure 1-16.
    Figure 1-16 Two stacks as a DAD relay agent 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 where the DHCP relay function is enabled.

  • 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 1-17. 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 is displayed for the stack. You only need to configure an IP address for this management interface.

    Figure 1-17 DAD through management interfaces
    As shown in Figure 1-18, 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 1-18 DAD through directly connected management interfaces

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.

Enhanced Stack DAD

The enhanced stack dual-active detection (DAD) function enhances the DAD mechanism and improves the convergence performance in case of a stack fault or recovery. Enhanced stack DAD has the following enhancements:
  • If a stack splits, the interval for sending DAD packets is shortened.
  • Comparing the physical status of uplink ports on stack members is added to the DAD competition rules. The stack master is elected according to the following rules in the listed order (the election ends when a winning switch is found):
    1. Physical status of uplink ports: The switch on which uplink ports are not all Down wins. For example, if the uplink ports of SwitchA are not all Down but the uplink ports of SwitchB are all Down, SwitchA wins the competition and becomes the stack master.

    2. Stack priority: The switch with a higher stack priority wins.

    3. MAC address: The switch with a smaller MAC address wins.

Updated: 2019-04-03

Document ID: EDOC1100075367

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