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Understanding the Chassis Cluster Data Plane

The data plane software, which operates in active/active mode, manages flow processing and session state redundancy and processes transit traffic. All packets belonging to a particular session are processed on the same node to ensure that the same security treatment is applied to them. The system identifies the node on which a session is active and forwards its packets to that node for processing. (After a packet is processed, the Packet Forwarding Engine transmits the packet to the node on which its egress interface exists if that node is not the local one.)

To provide for session (or flow) redundancy, the data plane software synchronizes its state by sending special payload packets called runtime objects (RTOs) from one node to the other across the fabric data link. By transmitting information about a session between the nodes, RTOs ensure the consistency and stability of sessions if a failover were to occur, and thus they enable the system to continue to process traffic belonging to existing sessions. To ensure that session information is always synchronized between the two nodes, the data plane software gives RTOs transmission priority over transit traffic.

Understanding Session RTOs

The data plane software creates RTOs for UDP and TCP sessions and tracks state changes. It also synchronizes traffic for IPv4 pass-through protocols such as Generic Routing Encapsulation (GRE) and IPsec.

RTOs for synchronizing a session include:

Session creation RTOs on the first packet
Session deletion and age-out RTOs
Change-related RTOs, including:
- TCP state changes
- Timeout synchronization request and response messages
- RTOs for creating and deleting temporary openings in the firewall (pinholes) and child session pinholes

Understanding Data Forwarding

For Junos OS, flow processing occurs on a single node on which the session for that flow was established and is active. This approach ensures that the same security measures are applied to all packets belonging to a session.

A chassis cluster can receive traffic on an interface on one node and send it out to an interface on the other node. (In active/active mode, the ingress interface for traffic might exist on one node and its egress interface on the other.)

This traversal is required in the following situations:

When packets are processed on one node, but need to be forwarded out an egress interface on the other node
When packets arrive on an interface on one node, but must be processed on the other node
If the ingress and egress interfaces for a packet are on one node, but the packet must be processed on the other node because its session was established there, it must traverse the data link twice. This can be the case for some complex media sessions, such as voice-over-IP (VoIP) sessions.

Understanding Fabric Data Link Failure and Recovery

Note: Intrusion Detection and Prevention (IDP) services do not support failover. For this reason, IDP services are not applied for sessions that were present prior to the failover. IDP services are applied for new sessions created on the new primary node.

The fabric data link is vital to the chassis cluster. If the link is unavailable, traffic forwarding and RTO synchronization are affected, which can result in loss of traffic and unpredictable system behavior.

To eliminate this possibility, Junos OS uses fabric monitoring to check whether the fabric link, or the two fabric links in the case of a dual fabric link configuration, are alive by periodically transmitting probes over the fabric links. Using this process, Junos OS detects fabric faults and disables one node of the cluster. It determines that a fabric fault has occurred if a fabric probe is not received but the fabric interface is active.

To recover from this state, you must reboot the disabled node. When you reboot it, the node synchronizes its state and RTOs with the primary node.

Note: If you make any changes to the configuration while the secondary node is disabled, execute the commit command to synchronize the configuration after you reboot the node. If you did not make configuration changes, the configuration file remains synchronized with that of the primary node.

Note: On SRX1400, SRX3400, SRX3600, SRX5600, and SRX5800 devices, fabric monitoring is disabled by default.