MTU on RB250GS?

SwOS
1

Can it be changed? Where? Guess I just need more 4 bytes to do Q-in-Q with 1500…

2

SwOS v1.2 supports up to 9000 byte frames, so there should not be any problems.

3

I see… Maybe isn’t a problem at all, but have a look on this picture:

Inside VLAN20 there is another VLAN with a different VLAN ID and “use-service-tag=yes”. Traffic seems flowing, but Im not sure if it is all ok. When I ping from RouterOS up to 1496 bytes it ping ok, but there are timeout when it is 1500. It should ping OK with 1500 till so fragment. Why it does NOT ping with 1497-1500 bytes then? Guess I will need to debug these packets…

edit
Found that interface ethernet where is installed the VLAN20 increases rx errors when pinging with 1500 bytes, pings 1496 long and lower do not increase rx errors.
edit

edit2
Interesting (just dunno why), pinging from 28 (Mikrotik minimum size) to 42 bytes, the output packet remains 56 bytes long always. I wonder if “packet size” on winbox means really it or some “payload”. Also, response packets (echo reply) are proportional to request packets nominal size i.e: echo request size 56 with payload 41 down to 28 (things you can change the size) have a answer packet 55 bytes long down to 42. Is that a normal standard or just a MT implementation behavior?

28 bytes echo request (packet size 56):

000c 422d b7ca 001d 0924 65b8 0800 4500 ..B-…$e…E.
001c db17 0000 ff01 b7c5 0a80 0a01 0a80 …
0a03 0800 2dd2 fc27 ce05 0000 0000 0000 …-..'…
0000 0000 0000 0000 …

42 bytes echo request (packet size 56):

000c 422d b7ca 001d 0924 65b8 0800 4500 ..B-…$e…E.
002a dd42 0000 ff01 b58c 0a80 0a01 0a80 .*.B…
0a03 0800 863f fc27 b108 3165 b3aa 8797 …?.'..1e…
185e b71f b076 d7f3 .^…v..

edit2

4

250GS can forward jumbo frames, but its CPU cannot receive ethernet frames bigger than 1518 bytes. Ethernet frame size is 1522 bytes, if you use double-tagging and 1500 bytes of IP load.



5

Ah, thank you very much. Then why cannot ping with 1500, but just 1496? Let me do some lab… I will post soon enough.

6

Because ethernet header (14 bytes) + two VLAN tags (2*4=8 bytes) + 1500 payload exceeds 1518 bytes and gets dropped.

7

Because ethernet header (14 bytes) + two VLAN tags (2*4=8 bytes) + 1500 payload exceeds 1518 bytes and gets dropped.

I see… Based on the picture above Ive made some lab by just adding a RB433AH on any ports between 2 and 5 and here is the results:

  • With a valid IP configuration on VLAN20 I can ping (from RouterOS PC) with 65535 bytes long OK and 1500 bytes long OK with “don’t fragment” set. That seems to be fair enough;
  • With another VLAN_X (use-service-tag=yes) inside that VLAN20 and a valid IP configuration on VLAN_X I can ping (from RouterOS PC) with 1496 bytes long ONLY, anything bigger simply does not reach VLAN_X interface on that RB433AH and also increment RX errors on that RouterOS PC interface.

Then Ive set that VLAN_X with MTU of 1496 and and finally fragmentation works but I may have problems with tcp mss, right? From where that size of 1518 you mention comes from? Thought L2MTU was 1522… So… with such a size of 1522 L2MTU (physical interface) shouldn’t it work at all?

8

I found the issue. Test labs Ive made I took into account those 1522 bytes L2MTU from RB433AH and works very well all that setup described above. Unfortunately, the PC Ive used in production (a DELL ones) is only 1518 indeed.

Thank you for your attention.
Ozelo

9

Dear Ozelo,

I am having the exact same issue but I am using RB2011il

I am using service tag to carry innervlan as well.

I can ping up to 1496

May I ask what I need to do?