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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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Principles

Principles

This section describes concepts and mechanisms of stacking technology.

Concepts

Figure 2-2 shows the roles and related concepts in a stack.
Figure 2-2 Roles and concepts in a stack
  • Roles

    Switches that have joined a stack are member switches. Each member switch in a stack plays one of the following roles:
    • Master switch

      The master switch manages the entire stack. A stack has only one master switch.

    • Standby switch

      The standby switch is a backup of the master switch. When the master switch fails, the standby switch takes over all services from the master switch. A stack has only one standby switch.

    • Slave switch

      A slave switch forwards service traffic. The more slave switches in a stack, the higher forwarding performance the stack can provide. Apart from the master and standby switches, all the other switches in a stack are slave switches.

  • Stack domain

    After switches are connected using stack links and set up a stack, they form a stack domain. Multiple stacks can be deployed on a network to support various applications. These stacks are identified by their domain IDs.

  • Stack member ID

    Stack member IDs are used to identify and manage member switches in a stack. Each member switch in a stack has a unique member ID.

  • Stack priority

    The stack priority of a member switch determines the role of the member switch in role election. A larger value indicates a higher priority and higher probability that the member switch is elected as the master switch.

  • Physical member port

    After the mode of a physical port is set to stack, the port becomes a physical member port. Physical member ports are used to connect stack member switches.

  • Stack port

    A stack port is a logical port exclusively used for stacking and includes several physical stack ports. Multiple physical member ports can be added to a stack port to improve stack link bandwidth and reliability.

    Each switch supports two stack ports, named Stack-Portn/1 and Stack-Portn/2, where n is the stack member ID of the switch.

Stack Setup

A stack is set up after the following stages:
  1. Physical connection setup: When multiple switches are connected in a specific topology according to network requirements, a stack network is established.

  2. Master election: Member switches exchange stack competition packets and elect a master switch according to master election rules.

  3. Topology collection: The master switch collects information about all the member switches and calculates the topology. If member switches have the same stack ID, the standby switches restart repeatedly.

  4. Running: The master switch synchronizes the topology of the entire stack to all the member switches and selects a standby switch.

Physical Connection Setup
Two stack topologies are available: chain topology and ring topology, as shown in Figure 2-3. Table 2-1 compares the two stack topologies in terms of reliability, link bandwidth utilization, and convenience of cable connections.
Figure 2-3 Stack topologies
Table 2-1 Comparison between stack topologies

Topology

Advantage

Disadvantage

Usage Scenario

Chain topology

Supports long-distance stacking because the first and last member switches do not need to be connected by a physical link.

  • Low reliability: If any stack link fails, the stack splits.

  • Low stack link bandwidth efficiency: The entire stack relies on a single path.

A chain topology is recommended when member devices are far from one another and a ring topology is difficult to deploy.

Ring topology

  • High reliability: If a stack link fails, the topology changes from ring to chain, and the stack can still function normally.

  • High link bandwidth efficiency: Data can be forwarded along the shortest path.

The first and last member switches need to be connected by a physical link, so this topology is not applicable for long-distance stacking.

A ring topology is recommended when member switches are located near one another, because this topology has higher reliability and link utilization.

Role Election

After a stack is set up, member switches exchange stack competition packets to elect a master switch. The member switches compare the following items in the listed order to elect the master switch (the election ends when a winning switch is found):

  1. Running status: The switch that starts first becomes the master switch.

  2. Stack priority: The switch with the highest stack priority becomes the master switch.

  3. Software version: The switch running the latest software version becomes the master switch.

  4. Bridge MAC address: The switch with the smallest bridge MAC address becomes the master switch.

    During the delivery of a device, 2, 8 or 16 MAC addresses are allocated to the device, among which the smallest MAC address becomes the bridge MAC address.

Topology Collection

After a master switch is elected, it collects information about all the member switches and calculates the topology. If member switches have the same stack ID, the standby switches restart repeatedly.

Stable Running
After the master switch completes topology calculation, it synchronizes the topology of the entire stack to all the member switches and selects a standby switch. The master switch compares the following items of member switches in the listed order to select the standby switch (the election ends when a winning switch is found):
  1. Stack priority: The switch with the highest stack priority becomes the standby switch.

  2. MAC address: The switch with the smallest MAC address becomes the standby switch.

Software Version Synchronization

A stack supports software version synchronization among the member switches. The member switches do not have to run the same software version, and they can set up a stack as long as their software versions are compatible with each another. If software version running on a member switch is different from that on the master switch, the member switch downloads the system software from the master switch, restarts with the new system software, and rejoins the stack.

Configuration File Synchronization

A stack uses a strict configuration file synchronization mechanism to ensure that the member switches work like one device.

  • When a stack is set up, each member switch starts with its own configuration file. After switches start, the standby and slave switches combine their stack configurations into the configuration file of the master switch to form the configuration file of the stack system.

  • When the stack is running normally, the master switch manages the entire stack, and synchronizes configurations made by users to the other switches in real time to maintain configuration consistency on all the member switches.

The configuration file synchronization mechanism ensures that the member switches save the same configuration file. If the master switch fails, other member switches can provide services using the same configuration file.

Configuration Combination and Conflict Detection

Configuration Combination

Stack configuration on a switch is saved in the configuration file. When a stack is set up, the standby and slave switches combine their own stack configurations with that of the master switch. The configuration combination rules are as follows:
  • The standby and slave switches combine their stack configurations with that of the master switch, including the stack attribute configuration, stack port configuration, and 40GE port split configuration. If the master switch has the offline stack configurations of the standby and slave switches, the stack configuration of the master switch takes effect.

    As shown in Figure 2-4, SwitchA and SwitchB in a stack combine their port configurations. Port 10GE2/17/5 on SwitchA is configured with common services and the configuration conflicts with the port configuration of SwitchB. Because SwitchA is the master switch, the port configuration of SwitchA takes effect.

  • After a stack is set up, the standby and slave switches synchronize their configuration files with the configuration file of the master switch to keep the same configuration with the master switch.

    As shown in Figure 2-4, SwitchB synchronizes its configuration file with that of SwitchA after the stack is set up.

Figure 2-4 Port configuration combination

Configuration Conflict Detection

A configuration conflict may occur if the master switch has offline configurations made for the standby and slave switches, which may cause a stack setup failure. A configuration conflict occurs in the following situations:
  • When member switches combine their physical member port configurations, the number of physical member ports in a stack port exceeds the limit.

  • All physical member ports on the standby and slave switches have the Shutdown configuration on the master switch or have configuration conflicts with the stack system.

  • A stack contains physical member ports of different types.

  • All physical member ports are added to different stack ports.

When any of the preceding conflicts occurs, the standby and slave switches cannot set up a stack with the master switch. In this case, modify the configuration of the master switch or the standby and slave switches to avoid configuration conflicts, and then restart the switches.

Stack Management

After a stack is set up, the member switches are virtualized into one device on the network. The management, login, and access methods are all different from those used on a single switch.

Member Switch Management

Member switches in a stack are managed on a per-slot basis and are identified by stack member IDs. When using commands to configure and manage the member switches in a stack, you must specify their stack member IDs. For example, you can run display device slot 2 to view information about a member switch in a stack. Here, 2 is the stack member ID of this switch.

In a stack, interface numbers contain stack member IDs, in the stack member ID/subcard ID/port number format. For example, after a switch joins a stack and is assigned stack member ID 2, the first interface on the switch is numbered 10GE2/17/1.

Stack Login

After a stack is set up, the member switches are virtualized into one device on the network, and all resources on the member switches are managed by the master switch. You can log in to the stack from any member switch to manage and maintain the entire stack. When you log in to a stack, you actually log in to the master switch, regardless of what login method you use and which member switch you have logged in to.

You can log in to a stack using the following methods:
  • Local login: Log in through the console interface of any member switch.

  • Remote login: Log in through the management interface or another Layer 3 interface of any member switch, using remote login protocols such as Telnet and STelnet.

    NOTE:
    • After a stack is set up, the configuration file of the master switch takes effect in the stack. Therefore, you must specify the IP address of the master switch when logging in to the stack remotely.

    • If multiple management interfaces are available in a stack, only one management interface takes effect.

File System Access

To access the file system on a switch, you need to specify the root directory of the flash storage. The flash storage name on a standalone switch without the stacking function is different from that on a stack member switch.
  • On a standalone switch without the stacking function:

    flash: indicates the root directory of the flash storage on the switch.

  • In a stack:
    • flash: indicates the root directory of the flash storage on the master switch.

    • Stack member ID#flash: indicates the root directory of the flash storage on the standby switch or a slave switch. For example, 2#flash indicates the root directory of the flash storage on the member switch with stack member ID 2.

For more information about the file system, see section "File System Overview" in the CX11x&CX31x&CX91x Series Switch Modules Configuration Guide - Basic Configurations.

Inter-Device Link Aggregation and Local Preferential Forwarding

Inter-Device Link Aggregation

A stack supports inter-device link aggregation (Eth-Trunk). That is, Ethernet ports on different member switches can be bound to one Eth-Trunk. The Eth-Trunk link still works when a member switch or a member link in the Eth-Trunk fails, ensuring reliable data transmission. Inter-device link aggregation prevents single-point failures in a stack and greatly improves network reliability.

As shown in Figure 2-5, traffic sent to the core device on the network is equally distributed to member links of an Eth-Trunk set up between the stack member switches. When an Eth-Trunk member link fails, traffic on this link is distributed to the other link. This link backup mechanism improves network reliability.
Figure 2-5 Link backup through inter-device link aggregation
As shown in Figure 2-6, when a member switch in the stack fails, traffic is switched to the Eth-Trunk member link on the other member switch. This device backup mechanism improves network reliability.
Figure 2-6 Device backup through inter-device link aggregation
Local Preferential Forwarding

When an inter-device Eth-Trunk is configured in a stack, the stack uses the hash algorithm to select outbound interfaces in the Eth-Trunk. Therefore, traffic received on a member switch may be forwarded through the other member switch. Inter-device forwarding consumes bandwidth on stack links. As bandwidth provided by a stack cable is limited, this forwarding mode increases loads on stack cables and reduces forwarding efficiency. Local preferential forwarding can solve this problem. This feature ensures that traffic reaching the local switch is preferentially forwarded through a local interface. If the local outbound interface fails, traffic is forwarded through an interface on the other member switch.

As shown in Figure 2-7, SwitchA and SwitchB set up a stack, and their uplink and downlink interfaces are bundled to Eth-Trunk interfaces. Without the local preferential forwarding feature, traffic reaching SwitchA is load balanced between the Eth-Trunk member links. Some of traffic is forwarded through the stack cables and sent out from a physical interface on SwitchB. If local preferential forwarding is enabled, traffic reaching SwitchA is forwarded through a local physical interface.

NOTE:

This function is only valid for known unicast packets, and is invalid for unknown unicast packets, broadcast packets and multicast packets.

Figure 2-7 Local preferential forwarding

Joining and Leaving a Stack

Joining a Stack
Figure 2-8 illustrates how a new switch joins a running stack.
NOTE:
  • A switch can be added to a stack while it is powered on or off. This section describes how a member switch joins a stack after being powered off. For details on how a member switch joins a stack after being powered on, see Stack Merging.

  • It is not recommended to add a member switch to a stack while the power is on.

Figure 2-8 A new member joins a stack

To add a switch to a stack, perform the following steps:

  1. Examine the physical connections between the current stack member switches and determine where to connect the new member switch.

    • If the stack has a chain topology, add the new switch to either end of the chain to minimize the impact on running services.
    • If the stack has a ring topology, tear down a physical link to change the ring topology to a chain topology, and add the new switch to either end of the chain. Then connect the switches on both end to restore the ring topology.
  2. Complete stack configuration on the stack and new member switch and save the configuration.

  3. Power off the new member switch, connect it to the stack using stack cables, and power it on.

The new member switch joins the stack as a slave switch, and the original master and standby switches retain their roles. If the stack member ID of the new member switch conflicts with another member switch in the stack, the slave switches restart repeatedly.

Leaving a Stack
A member switch leaves a stack after it is disconnected from the stack. Depending on the role of the switch that leaves the stack, the stack is affected in the following ways:
  • When the master switch leaves the stack, the standby switch becomes the new master switch. It then recalculates the topology, synchronizes updated the topology to the other member switches, and selects a new standby switch. Then the stack enters the running state.
  • When the standby switch leaves the stack, the master switch selects a new standby switch, recalculates the topology, and synchronizes the updated topology to the other member switches. Then the stack enters the running state.
  • When a slave switch leaves the stack, the master switch recalculates the topology and synchronizes the updated topology to the other member switches. Then the stack enters the running state.
A member switch leaves a stack after you disconnect its stack cables and remove it from the stack.
  • After removing a member switch from a ring stack topology, use a stack cable to connect the two ports originally connected to this member switch to ensure network reliability.
  • In a chain topology, removing an intermediate switch causes the stack to split. Therefore, analyze services before removing a member switch from the stack to minimize the impact on services.

Stack Merging

As shown in Figure 2-9, two stacks in the running state can merge into one stack. After two stacks merge, the master switches of the two stacks compete to be the master switch of the new stack. The two switches compare the following items in the listed order to elect the master switch (the election ends when a winning switch is found):
  • Stack priority: The switch with a higher stack priority becomes the master switch.

  • Software version: The switch running a later software version becomes the master switch.

  • Bridge MAC address: The switch with a smaller bridge MAC address becomes the master switch.

    During the delivery of a device, 2, 8 or 16 MAC addresses are allocated to the device, among which the smallest MAC address becomes the bridge MAC address.

After the new master switch is elected, the member switches originally belonging to the same stack as this new master switch retain their roles and configurations, and their services are unaffected. Switches in the other stack restart and join the new stack as slave switches, and services on these switches are interrupted.
Figure 2-9 Two stacks merge
Stack merging occurs in either of the following situations:
  • A switch is configured with the stacking function and is connected to a running stack using a stack cable while the power is on.
  • A stack splits because a stack link or member switch fails. After the stack link or member switch recovers, the two stacks merge into one again.

The stack merging process is similar to the process when a new member switch joins a stack. For details, see Joining and Leaving a Stack. The master competition rules used in a stack merging process are the same as the master election rules used in a stack.

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 2-10, the original master switch (SwitchA) and standby switch (SwitchB) are in the same stack after the split. SwitchA deletes topology information related to SwitchC and SwitchD and synchronizes new topology information to SwitchB. After SwitchC and SwitchD restart, they set up a new stack.

    Figure 2-10 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 2-11, the original master switch (SwitchA) and standby switch (SwitchB) are in different stacks after the split. SwitchA specifies SwitchC as the new standby switch, recalculates the stack topology, and synchronizes new topology information to SwitchC. In the other stack, SwitchB becomes the master switch. It then recalculates the topology, synchronizes topology information to SwitchD, and specifies SwitchD as the new standby switch.

    Figure 2-11 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

    In direct mode, DAD is performed through dedicated direct links between member switches, as shown in Figure 2-12.
    Figure 2-12 DAD in direct mode
    The direct detection links can also be connected through an intermediate device, as shown in Figure 2-13. 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 CX11x&CX31x&CX91x Series Switch Modules Configuration Guide - Ethernet.
    Figure 2-13 DAD through direct links to an intermediate device
  • Proxy mode

    In relay mode, DAD detection is performed through an inter-device Eth-Trunk link connected to a relay agent, as shown in Figure 2-14. 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.
    NOTE:

    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.

    Figure 2-14 DAD in reply mode
    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 2-15.
    Figure 2-15 Two stacks as DAD relay agents of each other
    NOTE:

    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 2-16. 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.
    NOTE:

    In this mode, ensure that the management interfaces of all member switches have been assigned IP addresses, are connected to the management network, and can communicate with each other over the layer 2 network.

    Figure 2-16 DAD through 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 shut down and stop forwarding service packets.

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

  2. MAC address of the switch: The switch with a smaller MAC address wins.

After the stack links recover, the stacks merge into one. The switches in Recovery state restart and restore the shutdown 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.

Master/Standby Switchover

A master/standby switchover occurs in a stack when the master switch restarts or when a user runs the switchover command. Figure 2-17 illustrates how the roles of member switches change after a master/standby switchover.
Figure 2-17 Changes of member switch roles after a master/standby switchover
  1. The original standby switch becomes the master switch.
  2. The new master switch selects a standby switch.
  3. The original master switch rejoins the stack as a slave switch after it restarts.

Stack Upgrade

A stack can be upgraded using the traditional upgrade method (specify the next-startup files and restart the entire stack) or the fast upgrade:
  • Traditional upgrade method: You need to specify next-startup files and restart the entire stack. This method causes service interruption in a long time and is therefore not applicable to scenarios requiring short service interruption time.

  • Fast upgrade: This upgrade method provides a mechanism to minimize the service interruption time during software upgrade of stack member switches, reducing impact of the upgrade on services.

    NOTE:
    • Only the stack containing two member switches supports fast upgrade.

    • It is recommended that the upstream and downstream devices be connected to the stack through Eth-Trunk links to reduce the traffic interruption time during an upgrade.

    • If the current version does not support the Smart Channel function but the system contains mode and configuration files related to Smart Channels, fast upgrade is not supported.

    Figure 2-18 shows traffic forwarding during a fast upgrade. First, the standby switch restarts with the new system software. Data traffic is forwarded by the master switch in this period. After the standby switch is upgraded, it becomes the master switch and starts to forward data traffic. Then the original master switch restarts with the new system software. After the upgrade is complete, the original master switch becomes the backup switch in the stack.
    Figure 2-18 Traffic forwarding during a fast upgrade

    If the upgrade fails due to a stack link failure, the system software rolls back to the original version.

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Updated: 2019-08-09

Document ID: EDOC1000041694

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