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Hierarchy Level


Enable adaptive load balancing (ALB) for a VCF, and specify how ALB is implemented.


Starting in Junos OS Release 14.1X53-D46, 15.1R7, 16.1R6, 17.1R3, 17.2R2, 17.3R2, and 17.4R1, the ALB feature is deprecated. If fabric-load-balance is enabled in the configuration, delete the configuration item upon upgrading Junos OS.

When ALB is enabled, the Virtual Chassis ports (VCPs) are reset. Packets are dropped and might potentially arrive out of order for a brief period of time as a result of this VCP reset. Normal operation of the VCF resumes after the VCP reset with no further user action.


ALB is disabled, by default.

If you do not specify a mode when enabled ALB, ALB is enabled using flowlet mode with an inactivity timer of 16 microseconds.



Implement ALB by using flowlets.

When ALB is implemented using flowlets, traffic flows that enter the VCF are spliced into smaller flows—flowlets—and individually forwarded across the VCF to the same destination device over different paths when the inactivity time between packet bursts on the sending interface exceeds the user-configurable inactivity interval.

The inactivity interval is 16 microseconds by default, and can be configured using the inactivity-interval statement. You should configure the inactivity interval to ensure in-order packet delivery, so that overall performance is not negatively impacted by the packet reordering process at the receiving device. To ensure in-order packet delivery, the inactivity interval should be larger than the largest latency skew among all the paths in the VCF from any node to any other node.

Implementing ALB using flowlets is especially effective in environments that periodically experience extremely large traffic flows—elephant flows—that are substantially larger than the majority of other traffic flowing through the VCF. The VCF is better able to manage elephant flows by splicing them into smaller flowlets using ALB.


Implement ALB using per-packet mode.

When per-packet mode is enabled, the VCF forwarding algorithm dynamically monitors all paths in the VCF and forwards packets to destination devices using the best available path at that moment. Flows are reordered at the destination node when per-packet mode is used to enable ALB, so some performance impact due to packet reordering is experienced.

Required Privilege Level

system—To view this statement in the configuration.system-control—To add this statement to the configuration.

Release Information

Statement introduced in Junos OS Release 14.1X53-D10.

Statement deprecated starting in Junos OS Releases 14.1X53-D46, 15.1R7, 16.1R6, 17.1R3, 17.2R2, 17.3R2, and 17.4R1.