RB850gx2 MTU issue confirmed by me:)

Hardware MikroTik hardware general
1

Hi Guys,

As soon as I create an VLAN-interface on a port and assign an address to it I can no longer manage the router via Winbox. I can ping the address on the infercace though.

Sometimes I can’t even login to Winbox. When I can login, most tabs are empty (like the interfaces tab, vlans tab, firewall tab, …) and the router disconnects within minutes. I’m going to mail a supout.rif to support but I was wondering whether someone else has come accross this?

edit: when I connect via serial cable, all the config looks ok.

2

MTU problem? Can you ping over vlan with 1500 byte packets?

3

I’ll check that. I have not changed the L2MTU. Perhaps I should set it at 1504. Can’t check the defaults now as I’m not near the router

I believe to ping 1500 bytes you need to set the ping size to 1472 in windows? (8 bytes ICMP and 20 bytes ip header makes 1500). Right?

4

MRZ you were very right!

For the VLAN interface you can only set an L2MTU of 1502 bytes (could nog get it to set 1504 or more). The MTU on the VLAN interface has to lowered (from 1500) to 1498 bytes in order for this to work.
1502 bytes - 4 bytes for VLAN header = 1498 bytes.
This looks like an issue to me. The physical interface, ether1, has an L2MTU of 1506 bytes. And a MTU of 1500 bytes.

Contrasting this to a 450G the VLAN interface has a L2MTU of 1516 bytes and an MTU of 1500 bytes.
The physical interface, has a L2MTU of 1520 bytes and a MTU of 1500 bytes.

So it seems a VLAN interface follows this rule: L2MTU of VLAN interface = L2MTU of pshysical interface - 4 bytes for VLAN header.

5

So… 850Gx2 with VLANs is useless?

6

and with VPLS is also useless…

http://forum.mikrotik.com/t/rb850g-and-vpls-as-i-do-if-max-l2mtu-is-1506/82058/1

7

This is quite worrying. One of my primary uses was for VLAN.

Can anyone at MT confirm if this is fixable with firmware / software upgrade or is the board physically limited in this regard?

8

I’ve been told it is currently an accepted bug. There is another topic where this was told. I hope it gets fixed quickly too:)

9

I asked MT about this MTU limitation. They assured me that the hardware supports jumbo frames and MTU should be increased in a future ROS version.

10

Good news. Better than mine as now we know the limitation can be removed. I hope future means this year though :slight_smile:

11

Does the latest ROS update fix this?

12

It appears ROS 6.21.1 has bumped the L2 MTU to 1580.

13

Hmm. That should help with the VLAN and MPLS issues, but it still isn’t much of a bump, especially for hardware that supports “jumbo frames.”

– Nathan

14

I think it is sufficient for all WAN uses. Is there a way to discover whether a router honours the dont fragment flag?:slight_smile:

15

ping with DF flag

16

You can set Cisco’s and Junipers’s to ignore the DF flag and they will happily fragment away. I’ll take a wireshark capture on a windows PC connected to the WAN port. But thanks for fixing this so quickly!
If it is good, it must be said too!

17

Patience grasshopper. They will arrive when they are ready.

18

Should I take this as an indication that a “little birdie” told you that we can expect the L2MTU on this product to eventually go beyond even 1580? Is support for 9K frames on the horizon, perchance?

– Nathan

19

Hi Friends!

Could you please confirm that the MTU limitation is already fixed in last firmware versions?
What is the limit now?

Thank you so much