Real-time Synchronization Among All Units in a Stack

Hitless stacking requires the active controller, standby controller, and stack members to be fully synchronized at all times. This is accomplished by baseline and dynamic synchronization of all units in a stack.

When a stack is first booted and becomes operational, baseline synchronization occurs across all of the units in the stack. The active controller copies the current state of its CPU to all units of the stack, including the standby controller. The information received from the active controller is programmed locally in the hardware on all units. The information includes:

  • Startup and runtime configuration (CLI) - These files are copied to the standby controller only.
  • Layer 2 protocols - Layer 2 protocols such as STP, RSTP, MRP, and VSRP run concurrently on both the active controller and standby controller.
  • Hardware Abstraction Layer (HAL) - This includes the prefix-based routing table, next hop information for outgoing interfaces, and tunnel information.
  • Layer 3 IP forwarding information - This includes the routing table, IP cache table, and ARP table as well as static and connected routes. Layer 3 routing protocols are not copied to any of the units in the stack but remain in standby state on the standby controller until a switchover occurs. Peer adjacency is restored after a switchover. If BGP4 or OSPF graceful restart is enabled during a switchover, the standby controller (new active controller) initiates a graceful restart, and a new set of routes is relearned. The new set of routes will be the same as the old routes, unless the network has changed.
  • OSPFv2 and OSPFv3 non-stop routing support - For more information, refer to the RUCKUS FastIron Layer 3 Routing Configuration Guide.

When control protocols are synchronized and protocol synchronization timers have expired, the standby controller is placed in hot-standby mode, ready to take over as the active controller. If a switchover occurs, the standby controller takes over without interrupting data traffic.

After baseline synchronization, any new events that occur on the active controller are dynamically synchronized on the standby controller. Such events include:

  • CLI, HTTP, and SNMP configurations
  • Link events
  • Interrupts
  • Layer 2 and Layer 3 forwarding table updates
  • Dynamic user authentication updates such as 802.1X or multi-device port authentication

After a switchover, the new active controller receives updates from the stack members and sends verification information to the stack members to ensure that they are synchronized.

Note: The events described previously occur internally and do not create or affect the external network topology.
Parent topic: What Happens During a Hitless Stacking Switchover or Failover