Archer 2600 - sqm, issues on Telekom VDSL2

Installing and Using OpenWrt Network and Wireless Configuration
6

I see the base line RTT seems to be almost twice as high with the C2600 than with the wdr4300, but in both cases the latency under load seems to be pretty flat, which I would chalk up as success.

Not sure, it would be interesting to look at the CPU load on the router while you perform a speedtest. You might want to have a look at https://forum.openwrt.org/t/speedtest-new-package-to-measure-network-performance/24647/36 which introduced a packet that will run a speedtest from your router that will also monitor CPU load and CPU frequency. This might give an indication about your router potentially running out of steam. I will add that this will only ever average CPU load accumulated over 1 second blocks, so it will not show all load spikes << 1 second, which still might negative influence the sqm performance. There is also https://github.com/dlakelan/routerperf by @dlakelan but this is in early alpha stage...

Anyway to get all the bells and whistles that cake offers tested:
Here is my proposed replacement for your /etc/config/sqm to enable per-internal IP fairness, nat-lookup and ingress-awareness, this also will enable ECN on outbound traffic (since your uplink seems fast enough):

config queue
       option debug_logging '0'
        option verbosity '5'
        option qdisc_advanced '1'
        option squash_dscp '0'
        option squash_ingress '0'
        option ingress_ecn 'ECN'
        option egress_ecn 'ECN'
        option qdisc_really_really_advanced '1'
        option linklayer 'ethernet'
        option overhead '34'
        option linklayer_advanced '1'
        option tcMTU '2047'
        option tcTSIZE '128'
        option tcMPU '64'
        option linklayer_adaptation_mechanism 'default'
        option iqdisc_opts 'nat dual-dsthost ingress'
        option eqdisc_opts 'nat dual-srchost'
        option interface 'pppoe-wan'
        option download '91543'
        option enabled '1'
        option upload '41146'
        option qdisc 'cake'
        option script 'layer_cake.qos'

To change this

  1. stop the current sqm instance:
    /etc/init.d/sqm stop

  2. Edit /etc/config/sqm with the editor of your choice (I like nano, if not installed just run opkg update ; opkg install nano to get hold of an editor that is both less capable and more user-friendly than vi)

  3. Start sqm again:
    ``/etc/init.d/sqm start

  4. Check (and post) the output of:
    tc -s qdisc

Give this a try and report back any comments you might have.
"nat" will allow cake to get to the true internal and external addresses wich seems important for the ingress shaper
"dual-xxxhost" will make cake first try to split the available bandwidth even between all concurrently active hosts (the way configured here will try to give each internal address an equal share of the bandwidth, this mode while super simple often comes close enough to what people want so they stop searching for the last ounce of QoS detail)
"ingress" will instruct cake to not try the customary approach where a shaper tries to enforce its outgoing bandwidth, but rather its incoming bandwidth. A subtle difference that makes cake deal better with different number of flows on the ingress side.