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Configuration Guide - Device Management

S1720, S2700, S5700, and S6720 V200R011C10

This document describes the principles and configurations of the Device Management features, and provides configuration examples of these features.
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Stack Split and MAD

Stack Split and MAD

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, device roles are elected in either of the following ways:
  • If the original master and standby switches are in the same stack after the split, the master switch calculates the stack topology by deleting topology information related to the removed member switches, and then synchronizes updated topology information to the other member switches. When removed member switches detect that the timeout timer for stack protocol packets has expired, the switches restart and begin a new master election.

    As shown in Figure 9-7, 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 topology information to SwitchB and SwitchC. After SwitchD and SwitchE restart, they set up a new stack.

    Figure 9-7  Original master and standby switches in the same stack after a split
  • If the original master and standby switches are in different stacks after the split, the original master switch selects a new standby switch in its stack, calculates stack topology information, and synchronizes updated topology information to the other member switches. The original standby switch becomes the new master switch in its stack, and then calculates stack topology information, synchronizes stack topology information to other the member switches, and selects a new standby switch.

    As shown in Figure 9-8, the original master switch (SwitchA) and standby switch (SwitchB) are in different stacks after the split. SwitchA specifies SwitchD as the new standby switch, calculates stack topology information, and synchronizes topology information to SwitchD and SwitchE. In the other stack, SwitchB becomes the master switch. SwitchB then calculates topology information, synchronizes topology information to SwitchC, and specifies SwitchC as the new standby switch.

    Figure 9-8  Original master and standby switches in different stacks after a split

Multi-Active Detection

All member switches in a stack use the same IP address and MAC address (that is, the MAC address of the stack). Therefore, after a stack splits, more than one stack may use the same IP address and MAC address. To prevent this situation, a mechanism is required to check for IP address and MAC address collision after a split.

Multi-active detection (MAD) is a stack split detection protocol. When a stack splits due to a link failure, MAD provides split detection, multi-active handling, and fault recovery mechanisms to minimize the impact of a stack split on services.

MAD Detection Modes

MAD can be implemented in direct or relay mode. The direct and relay modes cannot be configured together in the same stack.
  • Direct mode

    In direct mode, stack member switches use direct links over common network cables as dedicated MAD links. When the stack is running normally, member switches do not send MAD packets. After the stack splits, the member switches send a MAD packet every 1s over the MAD link to check whether more than one master switch exists.

    In direct mode, stack member switches can be directly connected to an intermediate device or fully meshed to each other:
    • Directly connected to an intermediate device (Figure 9-9): Each member switch has at least one MAD link connected to the intermediate device.

    • Fully meshed to each other (Figure 9-10): In the full-mesh topology, at least one MAD link exists between any two member switches.

    The use of an intermediate device can shorten the MAD links between member switches. This topology applies to stacks where member switches are far from each other. The full-mesh topology prevents MAD failures caused by failures of an intermediate device, but full-mesh connections occupy many interfaces on the member switches. Therefore, this topology applies to stacks with a few member switches.

    NOTE:
    • After configuring MAD in direct mode on an interface, do not configure other services on the interface.
    • A maximum of eight direct MAD links can be configured between member switches to ensure reliability.
    • MAD packets are bridge protocol data units (BPDUs), so the intermediate device must be able to forward BPDUs. For details on how to configure this function, see Configuring Interface-based Layer 2 Protocol Transparent Transmission in "Layer 2 Protocol Transparent Transmission Configuration" in the S1720, S2700, S5700, and S6720 V200R011C10 Configuration Guide - Ethernet Switching.
    Figure 9-9  MAD through direct links to an intermediate device
    Figure 9-10  Full-meshed connections between member switches
  • Relay mode

    In relay mode, MAD relay detection is configured on an Eth-Trunk interface in the stack, and the MAD detection function is enabled on an agent. Every member switch must have a link to the agent and these links must be added to the same Eth-Trunk. In contrast to the direct mode, the relay mode does not require additional interfaces because the Eth-Trunk interface can run other services while performing MAD relay detection.

    In relay mode, when the stack is running normally, member switches send MAD packets at an interval of 30s over the MAD links and do not process received MAD packets. After the stack splits, member switches send MAD packets at an interval of 1s over the MAD links to check whether more than one master switch exists.

    You can use an independent relay agent (Figure 9-11) or use two stacks as each other's relay agents (Figure 9-12).

    NOTE:
    • The relay agent is a switch that supports the MAD relay function. Currently, all the S series switches support this function.

    • To implement MAD relay detection by using two stacks as each other's relay agent, configure different domain IDs for the two stacks. Member switches of a stack form a stack domain. A network may have multiple stack domains, with different domain IDs.

    Figure 9-11  Single switch as the MAD relay agent
    Figure 9-12  Two stacks as MAD relay agents of each other

    Each Eth-Trunk can have a maximum of eight member interfaces. Therefore, when a stack contains nine member switches, one Eth-Trunk cannot provide MAD links for all the member switches. In this case, configure multiple Eth-Trunks to ensure that a MAD link is available between every two member switches. As shown in Figure 9-13, Eth-Trunk1 provides MAD links for Switch1 through Switch8, Eth-Trunk2 provides MAD links for Switch2 through Switch9, and Eth-Trunk3 provides MAD links for Switch1 and Switch9.

    Figure 9-13  MAD relay detection on multiple Eth-Trunks

Multi-Active Handling

After a stack splits, the MAD mechanism sets the new stacks to Detect or Recovery state. The stack in Detect state still works, whereas the stack in Recovery state is disabled.

MAD handles a multi-active situation in the following way: When multiple stacks in Detect state are detected by the MAD split detection mechanism, the stacks compete to retain the Detect state. The stacks that fail in the competition enter the Recovery state, and all the physical ports except the excluded ports on the member switches in these stacks are shut down. The stacks in Recovery state no longer forward service packets.

Fault Recovery

After the faulty link recovers, the stacks merge into one in either of the following ways:
  • The stack in Recovery state restarts and merges with the stack in Detect state, and the service ports that have been shut down are restored to Up state. Then the entire stack recovers.
  • If the stack in Detect state is also faulty before the faulty link recovers, you can remove this stack from the network and start the stack in Recovery state using a command to switch service traffic to this stack. Then rectify the stack system fault and link fault. After the stack recovers, connect it to the network so that it can merge with the other stack.
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Updated: 2019-09-23

Document ID: EDOC1000178167

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