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Cisco 3850/3504/9120 slow mgig speeds


Just bought an cisco 3850 with mgig and upoe, because i want more throughput on wired and wireless.

Got the 3504 and 9120 connected to mgig on the 3850, set the speed to 5000 on the 3504 via the 3850, also set the speed to 2500 on the 9120 via the 3850.

3850 was set to system mtu 9020.

9120 was set to 160mhz.

But i dont see any inprovement at all on wireless.

Did also just buy an 2.5g mgig NIC for my pc, but i only went from 1g to 1.5g.

All test were done using iperf3 running on my server connected with 10g to 3850.

I'm uploading configs for 3850 and 3504.

i'm also uploading pictures with my results from iperf.

Picture 1-2: iphone 12 mini iperf3 download and upload

Picture 3: Dell latitude e7270 with intel ax200

Picture 4: Fujitsu p420 with Asus PCE-C2500

Picture 5: HP z620 with intel x520 10gig (not mgig)

Best greetings from Christian Josiassen.
picture 5picture 5


picture 4picture 4picture 1picture 1picture 2picture 2picture 3picture 3

13 Replies 13

Rasika Nayanajith
VIP Mentor VIP Mentor
VIP Mentor

WiFi perspective, there is a huge difference in Data Rate vs Throughput. Also, it all depends on client capability (the below page may help you to understand different clients' capability) 
If you want to test the best case scenario, there should be only 1 client (iPhone in your example) on your AP & it should get higher RSSI/SNR to achieve the MCS11 data rate (1.2Gbps - assuming 80MHz, 2 spatial streams with OFDM). Even such data rate, throughput value is normally 40-60% of data rate, as there is a lot of overhead in a wireless cell. Therefore getting 506Mbps (upload direction) indicate you are getting close to the best possible values. 

Download speeds are not exactly what you would like to be, I would do further testing by checking RSSI/SNR "show client detail <mac_addr>" CLI output tell what AP thinks client RSSI. If you can get those details from Apple client itself, that is what matters and that determines the MCS rates the client go with.

*** Pls rate all useful responses ***



What about wired?

Why do i get slow speeds.

Best greetings from Christian Josiassen.

Leo Laohoo
VIP Community Legend VIP Community Legend
VIP Community Legend

@ChristianSchmidtJosiassen wrote:
9120 was set to 160mhz.

That is not going to work well.  Dial it down.  20- or 40- Mhz is fine.  

  1. What firmware is the 3850 on?
  2. What firmware is the 3504 on?
  3. What wireless NIC driver is the AX200 on?


I know but i want to test it, because i just spend 2600 usd on the 3850. Why is it that people on YouTube buying small consumer switches get more throughput than i do.
Best greetings from Christian Josiassen.

@ChristianSchmidtJosiassen wrote:
Why is it that people on YouTube buying small consumer switches get more throughput than i do.

Bragging rights. 

No sane wireless network engineer would ever enable 80- or 160 Mhz in production.  Lab, maybe.  To brag, yes.  Production, no!


It’s a lab, is there any settings i’m missing?

Best greetings from Christian Josiassen.

Rich R
VIP Advocate VIP Advocate
VIP Advocate

You haven't answered Leo's questions about software and driver versions ...

TAC recommended codes for AireOS WLC's
Best Practices for AireOS WLC's
TAC recommended codes for 9800 WLC's
Best Practices for 9800 WLC's
Cisco Wireless compatibility matrix
Field Notice: FN-72424 Later Versions of WiFi 6 APs Fail to Join WLC - Software Upgrade Recommended
WARNING - see CSCwd37092 Throughput degraded after upgrading to code - 2800/3800/4800 series
- The fix for CSCwd37092 is now released in and
- For IOS-XE 17.3.6 select controller model, go to IOS XE Software AP Service Pack, select CSCwd40096 17.3.6 APSP2
Field Notice: FN-63942 Lightweight APs and WLCs Fail to Create CAPWAP Connections Due to Certificate
                      Expiration - Software Upgrade Recommended
Field Notice: FN-72524 - During Software Upgrade/Downgrade IOS APs Might Remain in Downloading State
                     After 4 Dec 2022 Due to Certificate Expiration - Fixed in and 17.3.6 APSP5 (APSP_CSCwd83653)
                     Also fixed in (8.5 mainline) and (8.5 IRCM) if you can't upgrade to 8.10
                     Note that and have been deferred (withdrawn) and are effectively unsupported by Cisco
Richard R

Leo Laohoo
VIP Community Legend VIP Community Legend
VIP Community Legend

@ChristianSchmidtJosiassen, raise a TAC Case and ask them to confirm if this is CSCvx27800.


Do i need a service contract to open a TAC case?

Best greetings from Christian Josiassen



VIP Advisor VIP Advisor
VIP Advisor

For the wired connection try an updated driver directly from the chip maker:

Multi-Gig cards are very driver sensitive. 

For iperf, also try it with TCP traffic, UDP is very hard on the CPU in some cases (CPU load would rise on one core up to 100%), while TCP allows many newer features lowering the CPU clock. iperf is usually single threaded, which is why only one CPU core gets loaded. I think there was an option for parallel streams and threads, to even out CPU load problems.


Thank you so much, now i can upload files to my server at 2.5gb, i have been struggling with this for quite some time.

But do you know what might be wrong with the 3504 and the 9120?

Best greetings from Christian Josiassen

I don't have them to test sadly. But in general is the hardware optimized
for multiple stream transmission. In other words, you'd probably need
multiple clients and multiple servers for the load balancing algorithm to
work correct.

Now to optimize single stream in the private home (or very small company
without neighbor Wi-Fi networks), you could run 80 or 160 MHz wide 5 GHz
channels (remember, you only have about three 160 MHz non-overlapping
channels in most regions and a single foreign 20 MHz channel can wreck a
complete 160 MHz one) with modern 2x2 clients like the Intel AX201 or newer
and should reach some 1,1 Gbps or even 1,4 Gbps. Please note that I never
tested this, but his would probably be about the maximum possible with a
single client in a perfect environment (no other Wi-Fi on 5 GHz nearby).
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