Cut-through switching — where the switch begins forwarding a frame as soon as the destination MAC address has been received (14 bytes for standard Ethernet, 18 bytes for 802.1Q-tagged frames) — has different implications for jumbo frames versus standard frames. In a store-and-forward switch, the entire frame must be received into the buffer before forwarding begins, introducing a serialization delay of T_ser = frame_size / link_rate. For a 9,000-byte jumbo frame at 100 Gbps, T_ser = 9,000 × 8 / 100 × 10^9 = 720 ns, versus 120 ns for a 1,500-byte standard frame. In a cut-through switch, the forwarding decision starts after the first 14-18 bytes, which takes only 1.44 ns at 100 Gbps — independent of the total frame size. This means cut-through switching eliminates the 600 ns serialization disadvantage of jumbo frames for the first-hop switching latency. However, cut-through switching cannot be used when the egress link is slower than the ingress link (a 100 Gbps ingress to a 40 Gbps egress) because the switch would outrun the egress buffer. The switch must fall back to store-and-forward when the egress rate is less than 100% of the ingress rate, and in this scenario jumbo frames add the full serialization delay at the egress side: T_egress_ser = 9,000 × 8 / 40 × 10^9 = 1.8 μs for a 9,000-byte frame versus 300 ns for a 1,500-byte frame — a 6× latency penalty. The MTU performance impact tool models this by accepting a per-port egress rate profile and computing the average per-hop latency as L_hop = (fraction_cut_through × 18/rate) + (fraction_SAF × frame_size/rate), where fraction_cut_through is the proportion of ports where egress rate ≥ ingress rate.

The shared buffer architecture of modern data center switches (Broadcom Tomahawk 5, Jericho 2, Marvell Teralynx 10) imposes a per-port buffer allocation that interacts pathologically with jumbo frames in congestion scenarios. Each port is allocated a minimum guaranteed buffer (typically 128-256 KB in a 32 MB shared buffer pool across 64 ports). When a jumbo frame arrives and the egress port's buffer is occupied by other traffic, the frame must wait in the ingress virtual output queue (VOQ). A single 9,000-byte jumbo frame occupies 7.1% of a 128 KB minimum buffer — versus 1.2% for a 1,500-byte standard frame. When 32 ports simultaneously send jumbo frames to the same congested egress port, the aggregate buffer demand is 32 × 9,000 = 288 KB, exceeding the 256 KB minimum buffer allocation and spilling into the shared buffer pool. If all 64 ports concurrently congest the same egress, the buffer demand is 64 × 9,000 = 576 KB — exceeding even the shared pool's dynamic allocation for a single port and causing frame drops even when the total switch buffer is not fully utilized. The M/G/1 queueing model for the shared buffer shows that the drop probability increases from P_drop_1500 ≈ 0.001% (1,500-byte frames, 1,024-port buffer capacity at 100% load) to P_drop_9000 ≈ 0.1% (9,000-byte frames, same conditions) — a 100× increase in drop probability that directly impacts TCP throughput via the Mathis equation. The tool's buffer model computes the per-port and aggregate drop probabilities as a function of the frame size distribution and the port load, enabling operators to set the per-port guaranteed buffer size (via the switch's buffer-profile CLI) to accommodate jumbo frames without sacrificing drop performance.

The interplay between jumbo frames and the RoCEv2's PFC (Priority Flow Control) pause mechanism introduces a headroom sizing challenge that is often overlooked in MTU planning. PFC headroom — the buffer space reserved to absorb in-flight traffic after a PFC pause frame is sent — is calculated as H = (T_pause_turnaround + T_cable_delay) × line_rate, where T_pause_turnaround includes the switch's internal processing delay (approximately 2-3 μs for modern ASICs) and the link partner's response time. At 100 Gbps with a 100-meter cable (T_cable_delay = 500 ns), the headroom is H = (2.5 μs + 0.5 μs) × 100 Gbps = 37.5 KB — sufficient for 25 standard frames but only 4 jumbo frames. If the headroom is configured for 8 jumbo frames (72 KB), the XOFF threshold must be set higher, consuming more buffer per priority class and reducing the number of priorities that can share the buffer. In a switch with 16 MB total buffer configured for 4 lossless priorities (4 MB each), reserving 72 KB for headroom leaves 4,024 KB for data — only a 1.8% overhead. But in a switch with 8 MB total buffer supporting 8 lossless priorities (1 MB each), reserving 72 KB for headroom on each priority consumes 576 KB total — 7.2% buffer overhead, and the remaining 944 KB per priority must accommodate the sum of all traffic bursts on that priority before PFC engages. The MTU performance impact tool's PFC headroom calculator takes the cable length, the line rate, the switch ASIC pause turnaround latency, and the maximum number of jumbo frames that must be absorbed during the pause turnaround, and it outputs the required headroom allocation and the resulting per-priority buffer available. This enables operators to determine whether their switch's total buffer is adequate for jumbo-frame-based lossless RoCE fabrics before deploying the 9,000-byte MTU configuration.