PPPoE MTU issue upgrading from 6.31/6.32.3 to 6.33.x !!! WARNING !!!

RouterOS General
1

Dear Mikrotik customers,
we have upgraded our PPPoE Core access concentrators from 6.31/6.32.3 to 6.33.x (example 6.33.5)

In this 6.33.x version Mikrotik moved from non-standard large PPPoE MTU support to *) RFC4638 (MTU larger than 1488).
Upgrading only the core, but not the client, TCP/MTU will be broken (about 30% of destination will be unreachable!).
Also upgrading only client, but not PPPoE access concentrator, will be MTU issues in connections.

Mikrotik support reply us two time: “Upgrade both ends to version 6.33.2+ and issue should be gone”, but this is impossibile…
How to “call” all customers (500/1000…) and ask to update to a new version of Mikrotik firmware?
In this case we need to wait all client upgrade before upgrade core/access concentrators.
Some customers may even refuse to upgrade or not be able
Also updated Mikrotik client (waiting all users upgrade) can’t work correctly if the access concentrator is not updated.
This setup/firmware is not backward compatible as you can see and we are in a stalemate with the next firmware updates.

We hope this message help other Mirkotik customers to wait or not upgrade to the new version.

Have a nice day

Sim

2

Dear users,
the problem is still present !!!

If you upgrade your Access Concentrator (PPPoE Server) for 6.32.4 (Bugfix only) to 6.35 (Current) about 30% of destination for all your PPPoE Client with “<=6.32.4” will be unreachable!!!

:confused: :confused: :confused:

3

I think you will only have a problem when you have deployed your routers with nonstandard config, forcing the
MTU of the PPPoE connection to 1500 or so. Is that the case?

And what do you want to happen now? Is it only an advise for the community?

4

Hello!

All my routers/clients are all with standard PPPoE MTU (1492)

MKT “SERVER” with PPPoE SERVER:

  • Service Max MTU and Max MRU → 1492
  • Profile “Change TCP MSS → no”

MKT “CLIENT” with PPPoE Interface:

  • General Max MTU and Max MRU → 1492
  • Profile “Change TCP MSS → yes”

but this standard setup is incompatible with PPPoE Server updated and PPPoE Client not updated to the last version.

5

I have not observed such problems, having first used 1492 and now 1500 for the MTU/MRU.
But I have seen others report problems, and the number 1488 (which I have never observed).
Probably there are other factors, e.g. MTU of the physical layer.