Understanding the Control Plane

The control plane software is an integral part of JUNOS software that is active on the primary node of a cluster. It achieves redundancy by communicating state, configuration, and other information to the inactive Routing Engine on the secondary node. If the primary Routing Engine fails, the secondary one is ready to assume control.

This topic includes:

• About the Control Link
• About Heartbeats
• About Control Link Failure and Recovery
• Related Topics

The control plane software:

• Runs on the Routing Engine and oversees the entire chassis cluster system, including interfaces on both nodes
• Manages system and data plane resources, including the Packet Forwarding Engine (PFE) on each node
• Synchronizes the configuration over the control link
• Establishes and maintains sessions, including authentication, authorization, and accounting (AAA) functions
• Manages application-specific signaling protocols
• Establishes and maintains management sessions, such as Telnet connections
• Handles asymmetric routing
• Manages routing state, Address Resolution Protocol (ARP) processing, and Dynamic Host Configuration Protocol (DHCP) processing

Information from the control plane software follows two paths:

• On the primary node (where the Routing Engine is active), control information flows from the Routing Engine to the local Packet Forwarding Engine.
• Control information flows across the control link to the secondary node's Routing Engine and Packet Forwarding Engine.

The control plane software running on the primary Routing Engine maintains state for the entire cluster, and only processes running on its node can update state information. The primary Routing Engine synchronizes state for the secondary node and also processes all host traffic.