A5 series max MTU?


#1

I’m not finding any documentation about the A5 MTU, specifically the A5-14. It appears (via testing) to be 1524, at least on the LAN.

Any plans to raise it higher? The B5s pass 3200, but they don’t have to maintain wifi compatibility — if that matters.

Thanks!


#2

Hi Jeremy,

We validated that A5/C5 can pass up to 3200 bytes in RFC tests. Would you please check to see if your router/switch is configured to support jumbo frames?

If so, and it doesn’t work, please provide a diagram to support@mimosa.co so we can try to replicate your test. It may be worthwhile to take the troubleshooting offline, and then we’ll summarize the outcome in our troubleshooting guide.


#3

Thank you for responding, Chris. Sorry for the delay.

In testing we were able to pass 3200 bytes. I could not replicate the results of the first testing, so I’m going to assume something else was wrong with the methodology. In all other previous and present testing, 3200 was and is working fine.


#4

Hi Jeremy,

Thanks for the update. We did perform additional characterization testing here to see what the problem might have been. The A5 passes traffic up to 3200 bytes fragmented. It will not pass frames larger than 1500 bytes if the control frame is marked “Don’t Fragment” (DF) as per RFC 791.


#5

Thanks for the update. We did perform additional characterization testing
here to see what the problem might have been. The A5 passes traffic up to
3200 bytes fragmented. It will not pass frames larger than 1500 bytes if
the control frame is marked “Don’t Fragment” (DF) as per RFC 791.

Ah ha!

This is good to know — it’s acting as a router (which fragments) instead of
as a bridge. Confusing, and well worth documenting.

Thanks for getting back to me.


#6

Hi Jeremy,

This behavior has changed in the latest beta (2.0.2 FCS-6), and we added mention of it in the A5 Roadmap which will become the release notes. All of the following traffic types are now able to pass through without fragmentation:

  • TCP
  • UDP
  • PPoE Session
  • VPN: GRE, ESP, AH

There are additional improvements currently scheduled for firmware 2.0.3 to allow all traffic/protocols up to 3200 bytes without fragmentation.


#7

Thanks for the roadmap link, Chris — I hadn’t seen that. Great to see where you’re going with it.