Adam,
General comment: The packets aren't actually being fragmented. Most TCP
packets have the Do-Not-Fragment bit set, all equipment will honor
(except VPNs). However, when using a Mikrotik as a PPPoE server, it
automatically sets up some IP Mangle rules that modify the TCP MSS
(maximum segment size) during TCP session establishment on any traffic
coming from the PPPoE client and adjusting the TCP MSS down at least 8
bytes to accommodate the smaller overall MTU.
Specific to your equipment, I would guess the UBNT edge router doesn't
support RFC4638, which specifically describes having an MTU larger than
1492 for PPPoE traffic. The Mikrotik does (unless you're on a REALLY old
version of RouterOS).
On 8/13/19 6:57 AM, Dennis Burgess via AF wrote:
If you can set the L2MTU on the UBNT radio to 1508 or better yet 1510, then you
should be able to specify the MTU of the PPPoE to 1508, then you should be
able to have a full 1500 byte packets without fragmentation. Of course, if
UBNT can't do this, I would recommend not using them, but I regress. Else you
will have to use a device that can do that without issues, or suffer
fragmentation, not somtghing that is uncommon. It really won't make much
difference in the grand scheme of things.
Dennis Burgess, Mikrotik Certified Trainer
MTCNA, MTCRE, MTCWE, MTCTCE, MTCINE, MTCSE, HE IPv6 Sage, Cambium ePMP Certified
Author of "Learn RouterOS- Second Edition"
Link Technologies, Inc -- Mikrotik & WISP Support Services
Office: 314-735-0270 Website: http://www.linktechs.net
Create Wireless Coverage's with www.towercoverage.com
-----Original Message-----
From: AF <af-boun...@af.afmug.com> On Behalf Of Ken Hohhof
Sent: Tuesday, August 13, 2019 7:16 AM
To: 'AnimalFarm Microwave Users Group' <af@af.afmug.com>
Subject: Re: [AFMUG] PPPoE MTU Newb confusion
I can't speak to the Ubiquiti equipment, but PPPoE adds 8 bytes of overhead.
So MTU will normally be 1492, and the PPPoE client will normally enforce this
by modifying MTU advertisements from the client side.
What you want to do seems comparable to carrying 8 passengers in an 8 seat van,
forgetting that the driver takes up 1 seat.
-----Original Message-----
From: AF <af-boun...@af.afmug.com> On Behalf Of Adam Moffett
Sent: Monday, August 12, 2019 11:22 PM
To: AnimalFarm Microwave Users Group <af@af.afmug.com>
Subject: Re: [AFMUG] PPPoE MTU Newb confusion
If it's not clear.....I want to carry a mother truckin 1500 byte packet from
the end user to the internet. I have full control of the path between the
pppoe client and server, so I didn't anticipate this being a problem, but
somehow it's a problem.
On 8/13/2019 12:04 AM, Adam Moffett wrote:
So I've never done much with PPPoE.
In my test setup I have a Mikrotik PPPoE Server set with "Max MTU" of
9216. This is a CCR with an actual max of 10,000+ bytes.
The PPPoE client is a Ubiquiti ER-X-SFP. The Ubiquiti client will not
allow me to set an MTU on the PPPoE interface higher than 1500. They
physical interface has an MTU of 2016 (maximum for this platform).
The active session on the mikrotik server reports "Actual MTU" of
1480, and fragments packets larger than that. Packets larger than 1492
are silently dropped.
There's a switch between the tik server and ubnt client, and it has
jumbo frames enabled and has an MTU of 9216.
So I'm 100% confused.
If the MTU on the client is 1500, where are the limits of 1480 and/or
1492 coming from?
Why in the ever loving hell does the Ubiquiti router not allow an MTU
higher than 1500 on the PPPoE client?
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