How to Compute Headroom for Lossless Ethernet Links (Part 2)

Fundamental timing concepts

Bit time (BT) is the reciprocal of the line rate. For a 10 GbE port BT = 0.1 ns; for 100 GbE BT = 0.01 ns. A 512‑bit transmission therefore takes 512 BT. The Ethernet inter‑frame gap (IFG) is 96 bytes (96 BT) and the preamble + SFD occupy 8 bytes (64 BT), giving a total header time of 64 BT.

Worst‑case delay components (Figure 7‑2)

D‑Max‑Frame‑Len : pause threshold is reached while a maximum‑size frame (9216 bytes) is still being transmitted. Delay = 9216 × 8 + 96 + 64 = 73 888 BT.

D‑Pause : transmitting a 64‑byte pause frame takes 512 BT; adding IFG and preamble/SFD adds 160 BT, total 672 BT.

D‑Intf : interface latency defined by IEEE 802.3. Upper‑bound values are 8192 BT (10 GbE), 6144 BT (25 GbE), 24576 BT (40 GbE), 122 880 BT (100 GbE).

D‑Cable : propagation delay in fiber ≈5 ns per metre. For a 1 m cable this is 5 ns → 50 BT on 10 GbE, 5000 BT for a 100 m link.

D‑Resp : receiver response latency after a pause is received. Upper‑bound values are 30 720 BT (10 GbE), 40 960 BT (25 GbE), 60 416 BT (40 GbE), 201 728 BT (100 GbE), 463 360 BT (400 GbE).

D‑Max‑No‑Drop‑Frame‑Len : worst‑case delay for the largest frame allowed in the no‑drop class (e.g., 2300 bytes for FCoE). Delay = 2300 × 8 + 96 + 64 = 18 560 BT.

Total headroom calculation

The total worst‑case delay for a 10 GbE link with a 100 m cable is the sum of the components above:

D‑Total = 73 888 + 672 + 8 192 + 5 000 + 8 192 + 30 720 + 18 560 + 5 000 = 150 224 BT

150 224 BT ÷ 8 ≈ 18.78 KB of headroom is required on the receiver side.

Buffer cells on Cisco Nexus switches

Cisco Nexus switches allocate ingress buffers in fixed‑size cells (commonly 416 bytes). The command

show hardware internal buffer info pkt-stats input

reveals the cell size. A 64‑byte frame consumes one cell, leaving 352 bytes unused; a 2300‑byte frame occupies six cells, with the sixth cell partially used (220 bytes). Because each cell must be allocated in whole, the raw headroom of 18.78 KB is multiplied by the cell‑to‑frame ratio (416 ÷ 64 ≈ 6.5), resulting in an actual buffer requirement of roughly 122 KB .

Practical configuration example (Cisco Nexus 93180YC‑FX, 10 GbE FCoE)

Buffer‑size = 104 000 bytes

Pause‑threshold = 20 800 bytes

Resume‑threshold = 19 136 bytes

Headroom = 104 000 − 20 800 = 83 200 bytes for a short‑distance (100 m) link.

For a 10 km link the buffer‑size can be increased to 166 400 bytes, giving

Headroom = 166 400 − 20 800 = 145 600 bytes

The additional headroom (62 400 bytes) is not explained solely by cable delay; the default 100 m thresholds are over‑provisioned and the switch architecture adds undocumented margin.

Failure handling

failure: Ingress buffer allocation failed for interface Ethernet1/8

Remediation typically involves reducing the number of PFC‑enabled ports, lowering the buffer allocation, or both.

Ethernet pause vs. Fibre Channel B2B primitives

Initial exchange : Fibre Channel B2B exchanges a primitive during link initialization; Ethernet PFC does not exchange buffer counts.

Link utilization : B2B primitives are primitive‑sized and add no overhead; a 64‑byte Ethernet pause frame adds up to 512 Mbps of overhead when sent at high rates.

Duration exchange : PFC encodes a pause duration; B2B does not.

Direction : A pause frame tells the sender to stop transmitting (Tx pause); a Fibre Channel B2B R_RDY indicates the receiver is ready (Rx B2B).

Priority Flow Control (PFC) frame format

PFC extends the IEEE 802.3x pause frame with an 8‑bit Class Enable Vector and eight 16‑bit quanta fields (one per traffic class). The vector selects which classes the pause applies to (e.g., 00001000 enables class 3). Each quanta value specifies how many 64‑byte units a class must pause.

Mapping traffic classes to the PFC Class Enable Vector

Layer 2 PFC : VLAN tagging (IEEE 802.1Q) provides a 3‑bit Priority Code Point (PCP) that maps directly to one of the eight PFC classes. The VLAN ID (12 bits) can identify up to 4096 VLANs, but only eight CoS values are usable for PFC.

Layer 3 PFC : In routed environments the DSCP field in IPv4/IPv6 headers is used to map traffic to PFC classes. This allows PFC to operate across IP networks where VLAN headers may be absent.

Consistent mapping of VLAN ID/PCP or DSCP to the Class Enable Vector must be applied on both end‑devices and switches (DCBX or SDN can automate the synchronization).