topic Re: Download Speed Fluctuations on Cisco WLC 3504 in Wireless

Download Speed Fluctuations on Cisco WLC 3504

berni77at — Wed, 06 Aug 2025 07:12:05 GMT

Hi all,

I’ve taken over a customer environment using a Cisco WLC 3504 (running 8.10.196.0) and AIR-AP1852I-E-K9 access points.

After replacing the existing switches (previously Cisco) with new ones (likely Huawei or similar), we started noticing severe download performance fluctuations over Wi-Fi. Upload speeds are stable (~70 Mbps), but download speeds frequently drop to ~10 Mbps or lower — the issue is not tied to a specific AP and seems to "wander" between access points.

So far I’ve verified:

CAPWAP MTU is fine:
ping WLCip size 1476 df-bit succeeds — so no fragmentation issues.
TCP MSS is set to 1250, which should be safe.
AP hardware is not at fault: I’ve swapped APs, and the issue follows the location/network rather than the device.

Current suspicions:

Spanning Tree Topology Changes (TCs) fixed
LACP or trunk misconfiguration no problem
CDP/LLDP or native VLAN mismatch, no

My Questions:

Has anyone experienced similar behavior after replacing switches?
Are there any known compatibility issues between non-Cisco switches (Huawei currently installed) and Cisco APs or WLC 3504 regarding MTU, CAPWAP, or STP?
Any specific best practices when using third-party switches with Cisco wireless gear?

Appreciate any advice or pointers to troubleshoot this further.

Thanks in advance!

BR Bernhard

Re: Download Speed Fluctuations on Cisco WLC 3504

Chloeharper — Wed, 06 Aug 2025 07:15:31 GMT

I run into something similar when Cisco APs/WLCs were connected to non-Cisco switches not always a vendor issue, but certain defaults can cause throughput weirdness. A few things I’d check:

Duplex/Speed Mismatch – Make sure all AP uplinks are hard set or negotiated cleanly at 1Gbps full duplex on both ends. Some Huawei models have slightly different auto-negotiation behavior.
QoS Trust Settings – Cisco APs expect certain DSCP handling. If the new switches are remarking or dropping priority tags, you might see performance drops on download.
Multicast/Broadcast Handling – Enable IGMP snooping and make sure multicast packets aren’t being flooded unnecessarily, as that can seriously affect download performance over Wi-Fi.
WMM Settings – On the WLC, confirm that WMM is enabled for the WLAN. If the switch is not handling tagged frames as expected, this can reduce effective throughput.
CAPWAP Path – If the AP’s CAPWAP traffic is taking a different route due to VLAN changes or STP root shifts, you might be adding jitter/latency spikes.

If possible, run a continuous iperf3 test both up and down between a wired client and a Wi-Fi client, while watching for errors, drops, or interface counters incrementing on the switch and AP ports. That’ll tell you if the problem is physical/interface level or higher up the s

Re: Download Speed Fluctuations on Cisco WLC 3504

berni77at — Wed, 06 Aug 2025 07:40:07 GMT

1. where can is et ap to static 1 gig?

2. Currently Accessportport link-type access
port default vlan 99
stp edged-port enable
lldp mdn enable txrx
trust dscp

3. IGMP is not enabled. thought for capwap u dont need it

4. WMM is enabled

5. Capwap is layer 2 only

BR
Bernhard

Re: Download Speed Fluctuations on Cisco WLC 3504

srimal99 — Wed, 06 Aug 2025 07:47:55 GMT

If no issues were presented prior to switch refresh, then you can possibly look on wired side of the network. Always worth check from layer 1 and upwards.Physical cabeling , connectivity, duplex mismatches, in addition how have you set up qos marking, worth checking with vendor document for management traffic classification.

Re: Download Speed Fluctuations on Cisco WLC 3504

Mark Elsen — Wed, 06 Aug 2025 07:55:38 GMT

- @berni77at FYI : https://www.cisco.com/c/en/us/support/docs/wireless/wireless-lan-controller-software/220877-troubleshoot-wireless-speed-issues.html

Re: Download Speed Fluctuations on Cisco WLC 3504

MHM Cisco World — Wed, 06 Aug 2025 08:10:54 GMT

there is no compatibilty need between SW and WLC
I think you face some issue with SW itself not WLC

MHM

Re: Download Speed Fluctuations on Cisco WLC 3504

PSM — Wed, 06 Aug 2025 12:26:43 GMT

@berni77at In addition to the suggestions above you can connect a wired client as well on an adjacent wired port directly on the switch. Then you can start download simultaneously from wired and wireless. With this simple test you should be able to get more information about the issue.

Re: Download Speed Fluctuations on Cisco WLC 3504

JPavonM — Wed, 06 Aug 2025 13:26:17 GMT

Have you performed such throughput tests agains a local iPerf server?

Notice that performance issues impacting throughput are not only tied to your wireless/wired infrastructure, as things like shared RF medium (APs using same channels), channel congestion (2.4 vs 5 GHz band congestion; too many users and offenders using the same channel, from your infra and other infras), channel interference, noise levels, wireless adapter drivers, QoS policies in your WAN/LAN/WLAN, local router load, local and remoter WAN load, remote server load, ... all of that impacts throughput.

The best way for you to track the issues agains a potential defect in the AP code is to make the measurements against a local server and use an AP configured in a channel that no other AP is using in the vicinity.

Re: Download Speed Fluctuations on Cisco WLC 3504

eglinsky2012 — Fri, 08 Aug 2025 15:22:10 GMT

Since the issue started after swapping the switch, it seems like that would be related to the switch or its connections rather than RF, AP, client, etc.

Is the new switch performing any routing or is it all L2?

Check the uplinks and downlinks connected to the new switch for errors (CRC, input, etc.). And check the upstream switch/router for errors on its downlink(s) to the new switch. This sounds like a cabling issue. Also worth checking on CPU/memory usage of the switch.

Re: Download Speed Fluctuations on Cisco WLC 3504

berni77at — Mon, 11 Aug 2025 05:22:21 GMT

After visiting the site and speaking with the customer—who also reported LAN performance fluctuations. The issue was caused by one of the SFP+ modules in an LACP uplink, which had a high RX power loss (in dB). This poor optical signal on a single LACP member was leading to packet drops and unstable LAN performance.

Many thx for your help

BR
Bernhard

Replacing the defective SFP+ module resolved the problem completely.