This KB article outlines a scenario where packets are dropped in the "Best-Effort" queue, disrupting traffic even though the link shows low bandwidth utilization. It explains how traffic shaping configurations can impose limits that contribute to these drops, and provides guidance on resolving the issue.
Here is the related COs configuration on the device:
set class-of-service forwarding-classes class FC_BE queue-num 0set class-of-service forwarding-classes class FC_AF1 queue-num 1set class-of-service forwarding-classes class FC_AF2 queue-num 2set class-of-service forwarding-classes class FC_AF3 queue-num 3set class-of-service forwarding-classes class FC_AF4 queue-num 4set class-of-service forwarding-classes class FC_EF queue-num 5set class-of-service forwarding-classes class FC_NC queue-num 6
set class-of-service scheduler-maps SCHEDULER-MAP forwarding-class FC_AF1 scheduler SCHED_AF1set class-of-service scheduler-maps SCHEDULER-MAP forwarding-class FC_AF2 scheduler SCHED_AF2set class-of-service scheduler-maps SCHEDULER-MAP forwarding-class FC_AF3 scheduler SCHED_AF3set class-of-service scheduler-maps SCHEDULER-MAP forwarding-class FC_AF4 scheduler SCHED_AF4set class-of-service scheduler-maps SCHEDULER-MAP forwarding-class FC_BE scheduler SCHED_BEset class-of-service scheduler-maps SCHEDULER-MAP forwarding-class FC_EF scheduler SCHED_EFset class-of-service scheduler-maps SCHEDULER-MAP forwarding-class FC_NC scheduler SCHED_NC
set class-of-service schedulers SCHED_BE transmit-rate percent 4set class-of-service schedulers SCHED_BE shaping-rate percent 4set class-of-service schedulers SCHED_BE priority low
set class-of-service schedulers SCHED_AF1 transmit-rate percent 1set class-of-service schedulers SCHED_AF1 shaping-rate percent 1set class-of-service schedulers SCHED_AF1 priority low
set class-of-service schedulers SCHED_AF2 transmit-rate percent 14set class-of-service schedulers SCHED_AF2 shaping-rate percent 14set class-of-service schedulers SCHED_AF2 priority low
set class-of-service schedulers SCHED_AF3 transmit-rate percent 34set class-of-service schedulers SCHED_AF3 shaping-rate percent 34set class-of-service schedulers SCHED_AF3 priority low
set class-of-service schedulers SCHED_AF4 transmit-rate percent 35set class-of-service schedulers SCHED_AF4 shaping-rate percent 35set class-of-service schedulers SCHED_AF4 priority low
set class-of-service schedulers SCHED_EF transmit-rate percent 10set class-of-service schedulers SCHED_EF shaping-rate percent 10set class-of-service schedulers SCHED_EF priority strict-high
set class-of-service schedulers SCHED_NC transmit-rate percent 2set class-of-service schedulers SCHED_NC shaping-rate percent 2set class-of-service schedulers SCHED_NC priority low
set class-of-service interfaces xe-0/0/0 scheduler-map SCHEDULER-MAPset class-of-service interfaces xe-0/0/0 shaping-rate 3g
The device is dropping packets in the "Best-Effort" queue on egress, even though overall throughput is very low compared to the interface’s 10 Gbps capacity:
root@user> show interfaces xe-0/0/0 extensive
Physical interface: xe-0/0/0, Enabled, Physical link is Up CoS queues : 10 supported, 10 maximum usable queues Traffic statistics: Input bytes : 5432817230822 13824752 bps Output bytes : 51029861134838 120025592 bps Input packets: 14051591216 5113 pps Output packets: 41934699699 12865 pps IPv6 transit statistics: Input bytes : 0 Output bytes : 0 Input packets: 0 Output packets: 0 Input errors: Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Policed discards: 0, L3 incompletes: 0, L2 channel errors: 0, L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0 Output errors: Carrier transitions: 3, Errors: 0, Drops: 366201423, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0 Egress queues: 10 supported, 9 in use Queue counters: Queued packets Transmitted packets Dropped packets 0 42174622171 41808420748 366201423 1 0 0 0 2 2475 2475 0 3 0 0 0 4 1344 1344 0 5 4995 4995 0 6 0 0 0 7 126306784 126306784 0 8 0 0 0 Queue number: Mapped forwarding classes 0 FC_BE 1 FC_AF1 2 FC_AF2 3 FC_AF3 4 FC_AF4 5 FC_EF 6 FC_NC 7 network-control 8 mcast CoS information: Direction : Output CoS transmit queue Bandwidth Buffer Priority Limit % bps % usec 0 FC_BE 4 120000000 r 0 low none 1 FC_AF1 1 30000000 r 0 low none 2 FC_AF2 14 420000000 r 0 low none 3 FC_AF3 34 1020000000 r 0 low none 4 FC_AF4 35 1050000000 r 0 low none 5 FC_EF 10 300000000 r 0 strict-high none 6 FC_NC 2 60000000 r 0 low none
The interface is lightly utilized relative to its 10 Gbps capacity, currently receiving approximately:
Despite this low throughput, there has historically been congestion and queue exhaustion on the default egress queue. The root cause is the "shaping‑rate" settings applied to the "Best-Effort" queue:
It’s important to note that the "transmit‑rate" defines the scheduler’s guaranteed bandwidth and minimum allocation during congestion, but it does not strictly cap traffic. If bandwidth is available, the queue can exceed its 4% allocation. On a 10 Gbps interface, 4% equals ~400 Mbps, and bursts above this threshold can still occur. However, the configuration line "shaping‑rate" percent 4 enforces a hard traffic shaper, effectively rate‑limiting the "FC_BE" queue to ~400 Mbps. Any traffic beyond this limit is buffered and eventually dropped once the queue fills, meaning short spikes—even if average traffic is below 400 Mbps—can still cause microbursts.
Although overall bandwidth utilization remains low and the link is not saturating capacity, the "shaping‑rate" is prematurely constraining traffic. To address this, you could consider:
Each of these workarounds will depend on the amount of traffic being handled, so it’s important to analyze the traffic patterns and bandwidth utilization first.