Community discussions

MikroTik App
 
adwlodaro
just joined
Topic Author
Posts: 3
Joined: Wed Feb 08, 2023 7:12 pm

physical VS logical interface MTU

Thu Jun 01, 2023 6:08 pm

Hi,

I have to admit at first that I don't fully understand the relationship between the MTU values of the physical and logical interface. Let's say this is my FTTH configuration:
- VLAN 40
- PPPoE MTU 1540 (PPP-Max-Payload Tag and LCP MRU show this value),
so I've setup my interfaces like this (on a CHR VM):
mtu_logical_physical.jpg
Can you explain me how and why does it work? It seems to my like PPPoE payload somehow doesn't count to the MTU physical interface? Shouldn't this PPPoE packets be dropped if they are larger than the MTU of their "master" physical interface ether1?
Thanks for any help!
You do not have the required permissions to view the files attached to this post.
 
User avatar
rextended
Forum Guru
Forum Guru
Posts: 11967
Joined: Tue Feb 25, 2014 12:49 pm
Location: Italy
Contact:

Re: physical VS logical interface MTU  [SOLVED]

Sat Jun 03, 2023 12:16 am

I do not know the precise model you have.
Assuming the ether1 max L2 MTU is 1596, and MAX L3 MTU is 1500,
The VLAN can reach the L2 MTU of 1592 (1596-4), so L2 1592 for PPPoE (if is enabled on both end) are possible.
But what you see on the list, on image, is the L3 MTU not the L2 MTU.

But I advise against in the most absolute way, unless there is a good reason with a precise background, set the L3 MTU to the internet greater than 1500,
so in your pppoe-client i suggest you set MTU and MRU to 1500 (is the L3 MTU) disabling the MRRU.
The L2 MTU is 1500 + all payload & co. of the pppoe and is automatic.

Do not mix L3 MTU and L2 MTU, you're lucky that your provider doesn't do like many others, limiting the L3 MTU to 1480 or 1492 when using pppoe.

Who is online

Users browsing this forum: baragoon, Google [Bot], onnyloh, sokalsondha and 52 guests