H-QoS Oversubscription on a 10 Gb/s Subscriber Port
A 7750 SR BNG serves 500 residential subscribers over a single 10 Gb/s port. The wholesale agreement sells three H-QoS tiers: 100 Platinum subscribers at 50 Mb/s CIR, 200 Gold at 20 Mb/s CIR, and 200 Silver at 5 Mb/s CIR. The sum of CIR is 10 Gb/s, exactly equal to port line rate, while peak PIR demand reaches 18 Gb/s. MPLS and Ethernet overhead reduce usable capacity to about 9.5 Gb/s. The port scheduler is configured with one default level and weights of 10/5/1; each subscriber has a parent scheduler but no explicit rate. At peak, Platinum subscribers report 35 to 40 Mb/s instead of 50 Mb/s, Silver latency reaches 120 ms, and port egress shows drops. Operations proposes reducing Silver PIR. Product insists that all CIR must be guaranteed because it is in the contract.
1. Assess whether the CIR guarantee is technically feasible and explain the reasoning.
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The contract may promise CIR, but the implementation cannot guarantee 10 Gb/s of CIR on a 9.5 Gb/s usable port. CIR is a scheduler commitment that depends on the port scheduler, subscriber parent scheduler, and queue configuration; it is not a separate physical channel. With sum CIR at line rate, any overhead, other traffic, or scheduler imperfection causes shortfall. The 1.8:1 PIR oversubscription is separately aggressive. Platinum at 35 to 40 Mb/s instead of 50 Mb/s is consistent with missing parent rates and default weights, but even perfect configuration cannot exceed port capacity. Reducing Silver PIR alone may reduce drops but will not create the missing 0.5 Gb/s; admission control or more capacity is required.
2. Propose the H-QoS and admission-control changes you would make.
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Configure explicit H-QoS: a scheduler policy with subscriber parent schedulers at the sold CIR and PIR, child queues for service classes, and a port scheduler with levels and weights that give Platinum priority while preventing starvation. Set parent rates so Platinum's aggregate has a reserved share; use shaping on Silver. Then perform admission control: either reduce sold CIR to fit within about 9.5 Gb/s with headroom, cap the number of Platinum subscribers, or add a second 10 Gb/s port or link bundle. If Silver PIR is reduced, verify it actually reduces port contention; it will not fix Platinum if the parent scheduler is unconfigured. Monitor per-subscriber CIR compliance and port scheduler drops.
3. If product insists on the current CIR promise, what alternatives can you offer?
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Offer alternatives rather than accepting an impossible guarantee: sell a statistical CIR with a documented oversubscription ratio, add capacity, or reclassify some Platinum subscribers to a lower CIR. Technically you cannot guarantee sum CIR above usable line rate; document the risk and the assumed traffic model. Use per-subscriber shaping and H-QoS to protect the higher tiers, and report actual delivered rate versus CIR. Do not accept a design that relies on average utilization and assumes congestion will not occur.