topic Re: Strange MTU behavior PPPOE in Firewall and Security Management

Strange MTU behavior PPPOE

RZomerman — Mon, 03 Feb 2020 09:06:10 GMT

Hi everyone,

I have a PPPOE connection to my telco which gives me an MTU of 1492.

However the device seems to be unable to process a packet >1472 bytes that I send out to the internet.

My setup is rather simple, ISPmodem->eth1-CP4400->Mngt-client

when the client is pinging to a public IP address, it all goes fine, as long as the packet size is smaller than 1473. I can send up to 1472, but as soon as I go over it packets are dropped. Which causes weird behaviors obviously.

I'm on R80.20 Build: 101

Any idea as to why packets >1472 are dropped? Is there a way to find out what is happening?

I had Junipers/USG's connected to the same link and those all work fine..

I don't have a support contract as this is my demo device to run some tests on at home..

Re: Strange MTU behavior PPPOE

RZomerman — Tue, 04 Feb 2020 12:32:04 GMT

fixed!

The trial license allowed me to update to 80.30 and it's working now..

Re: Strange MTU behavior PPPOE

Maarten_Sjouw — Tue, 04 Feb 2020 15:37:25 GMT

With the do not fragment bit set and an IP header of 20 bytes this would be expected behavior.

Re: Strange MTU behavior PPPOE

Timothy_Hall — Sun, 09 Feb 2020 16:03:16 GMT

It is possible that the ICMP packet somehow got tangled up with SecureXL under R80.20 since SecureXL does not support Point-To-Point links, but ICMP is never supposed to be accelerated by SecureXL at all so that seems consistent with some kind of bug that was fixed.