Testing QoS with iperf3 & DSCP — Let Aruba Prioritize

To test QoS you need two things: traffic marked with a DSCP value, and a switch that trusts and prioritizes that marking. iperf3 does the marking (via the IP ToS byte); an Aruba AOS-CX switch does the prioritizing. Here's the whole loop — short — with the one calculation you need and how to prove it worked.

01The loop: mark → trust → prioritize

Three steps, one line each: iperf3 stamps a DSCP on the packets, the switch is told to trust DSCP and map it to a queue, and you confirm the packets carried the mark and hit the right queue.

QoS test = iperf3 (mark DSCP) + Aruba (trust DSCP → queue) + verify (capture + queue counters)

02DSCP ↔ ToS ↔ CoS: the conversion table

iperf3 sets the full 8-bit ToS byte, but people think in DSCP (the top 6 bits). Convert with one operation — ToS = DSCP × 4 (shift left 2, because the low 2 bits are ECN). The full mapping, colored by service class (warmer = higher priority):

Class	DSCP	Dec	Binary	ToS dec	ToS hex	CoS	iperf3 `-S`
Network control	CS7	56	111000	224	0xE0	7	0xE0
Internetwork ctrl	CS6	48	110000	192	0xC0	6	0xC0
Voice (VoIP)	EF	46	101110	184	0xB8	5	0xB8
Signaling	CS5	40	101000	160	0xA0	5	0xA0
Interactive video	AF41	34	100010	136	0x88	4	0x88
Interactive video	AF42	36	100100	144	0x90	4	0x90
Interactive video	AF43	38	100110	152	0x98	4	0x98
Interactive video	CS4	32	100000	128	0x80	4	0x80
Streaming video	AF31	26	011010	104	0x68	3	0x68
Streaming video	AF32	28	011100	112	0x70	3	0x70
Streaming video	AF33	30	011110	120	0x78	3	0x78
Broadcast video	CS3	24	011000	96	0x60	3	0x60
Low-latency data	AF21	18	010010	72	0x48	2	0x48
Low-latency data	AF22	20	010100	80	0x50	2	0x50
Low-latency data	AF23	22	010110	88	0x58	2	0x58
OAM / network mgmt	CS2	16	010000	64	0x40	2	0x40
Bulk data	AF11	10	001010	40	0x28	1	0x28
Bulk data	AF12	12	001100	48	0x30	1	0x30
Bulk data	AF13	14	001110	56	0x38	1	0x38
Scavenger	CS1	8	001000	32	0x20	1	0x20
Best effort	DF / CS0	0	000000	0	0x00	0	0x00

Control
Voice
Signaling
Interactive video
Streaming video
Data
Bulk / scavenger
Best effort

ToS byte = DSCP × 4 · DS field = DSCP (6 bits) + ECN (2 bits) · CoS ≈ DSCP ÷ 8

03Mark the traffic with iperf3

Server on one host; client on the other, marking EF with -S 0xB8. Run a best-effort flow too, so you can compare:

# receiver (10.0.0.2)
iperf3 -s

# sender (10.0.0.1): EF-marked UDP, then plain best-effort
iperf3 -c 10.0.0.2 -u -b 100M -t 10 -S 0xB8      # DSCP 46 (EF)
iperf3 -c 10.0.0.2 -u -b 100M -t 10              # DSCP 0 (best effort)
# newer iperf3 also accepts:  --dscp 46   (or a name, e.g. --dscp ef)

-S is the ToS byte from the table above. That's the entire client-side story.

04Make Aruba prioritize it

By default AOS-CX trusts nothing (marks are ignored). Tell it to trust DSCP and map EF to the top local-priority (which the queue profile sends to the priority queue):

switch(config)# qos trust dscp                                  # honour incoming DSCP markings
switch(config)# qos dscp-map 46 local-priority 7 name EF-Voice  # DSCP 46 (EF) -> local-priority 7
# per-port override if you don't want it global:
switch(config)# interface 1/1/1
switch(config-if)# qos trust dscp

That's it: EF now rides local-priority 7 into the highest queue; everything at DSCP 0 stays best-effort.

05Verify it worked

On the wire — confirm the packets actually carry EF (ToS 0xb8) leaving the sender:

tcpdump -ni eth0 -v 'host 10.0.0.2 and udp' | grep -i tos

IP (tos 0xb8, ttl 64) 10.0.0.1.5xxxx > 10.0.0.2.5201: UDP, length 1450   <- 0xb8 = EF, good

On the switch — watch the per-queue counters; under congestion the EF queue keeps moving while best-effort drops:

switch# show qos dscp-map                 # confirm 46 -> local-priority 7
switch# show interface 1/1/1 qos          # per-queue tx / drops

DSCP  local-priority  name
46    7               EF-Voice
Queue 7:  tx 84213 pkts   drops 0        <- EF protected
Queue 0:  tx 61190 pkts   drops 3122     <- best-effort absorbs the loss

proof = capture shows tos 0xb8 + EF queue drops ≈ 0 while best-effort drops rise

06Where to go next

Scale the same recipe: mark AF41 (-S 0x88) for video alongside EF, congest the link, and watch each class land in its queue. On older ArubaOS-Switch (2930/5400) the equivalent is qos type-of-service diff-services to trust DSCP plus qos dscp-map <codepoint> priority <0-7>. Back to the Knowledge Base, or more Networking guides.