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petasis · ‎09-28-2022

Hi all,

I am facing a problem with my wired LAN. I have an RT-AX88U. Behind it I had a couple of switches, and everything was ok with my LAN, file transfers were ok (because the switches supported jumbo frames).

Recently, I replaced one of the switches with an RT-AX92U, connected with a new ethernet cable to the RT-AX88U (as an AIMESH node, since the RT-AX88U wifi signal is weak). And the problems started, jumbo frame support of my LAN was destroyed. All file transfers are SLOW, like 30% slower. And I have to disable jumbo frame support in all of my PCs, as the LAN was unreliable!

So, I started checking the two routers, just to find out that none of them supports jumbo frames! Both of them have an option in their "Switch control" tab in the setting, but enabling/disabling it has no effect.

I have conducted ASUS, but they provided no solution, just to "wait" for new firmware updates. A firmware update was released for RT-AX88U, but it does not fix the problem. No firmware update has been released for RT-AX92U for about 9 months.

What should I do? How can I fix the problem?

Its the first time I see a device with no support for jumbo frames (both routers say they support jumbo frames, but they don't).

jzchen · ‎09-30-2022

If that is the case put back the old switch with old Ethernet cable. Then can you just add the RT-AX92U very near the old switch with a short Ethernet cable and place it nearby?

petasis · ‎10-04-2022

No, this is not an option. Extra devices that consume power (they are always on), power costs money, and extra devices reduce the UPS runtime.
If I need to add the switch back, I will have to remove the router.

But the problem is that the specs say jumbo frames are supported by both routers, which is not true.
I would prefer this to be fixed, and advertise specs to match real ones.

jzchen · ‎10-05-2022

Recently, I replaced one of the switches with an RT-AX92U, connected with a new ethernet cable to the RT-AX88U (as an AIMESH node, since the RT-AX88U wifi signal is weak). And the problems started, jumbo frame support of my LAN was destroyed. All file transfers are SLOW, like 30% slower. And I have to disable jumbo frame support in all of my PCs, as the LAN was unreliable!

What I don't understand is how it was working fine then suddenly stopped supporting jumbo frames. You only changed one switch with an RT-AX92U. Are you confident the new cable is not defective?

I researched jumbo frames setting and it isn't clear to me that it is needed for high speed LAN....

Murph_9000 · ‎10-05-2022

petasis wrote:
But the problem is that the specs say jumbo frames are supported by both routers, which is not true.
I would prefer this to be fixed, and advertise specs to match real ones.

Where does it say in the specs that jumbo frames are supported? I don't see it on either of:

Beware also that there are different levels of jumbo frame support out there. Since they are not really necessary for typical consumer grade equipment, support for 9000 byte frames isn't particularly common in that segment, it's more of an enterprise network feature for data center and ISP networks. The feature on the routers, if present, may be just 1508 MTU baby jumbo frames on the WAN side to allow 1500 MTU over PPPoE, which is a more common and useful consumer level feature. Note also, that all devices on the same LAN need to support the same MTU, which is likely to be limited to 1500 on many consumer client devices.

I'd recommend against 9000 MTU on most small networks because of the extra pain and incompatibilities that come with it. If you really do need it for something like a fairly serious NAS/SAN setup, I would recommend using it on a separate storage LAN/VLAN and sticking to 1500 MTU for the general purpose LAN & Internet.

Broken LAN: no jumbo frames support...