Good evening!
Sorry for bad english
I have a task to change MTU on MikroTik ether1 interface from default 1500 bytes to 1700(for large packets through EoIP tunnel), i tuned it in interface settings and changed it on local switch(dlink dgs3620-28sc), but when i try to ping neighbor interface with no fragmentation it doesn’t work. After testing i saw, that i can ping only with ping packet size 1534 and below. Jumbo frames on local switch is enable. Now there is firmware v6.42.6 (stable), but i tried to upgrade. Besides i have an 951Ui model too and all works fine. Can you prompt me what can i do with that?
There is also L2 MTU on interfaces. You should try to change this one.
I also changed it. Except bridge.
Which interface is “neighbor interface”? The switch itself or some other adjacent machine? The problem with setting larger MTU is that all devices in same L2 (broadcast) domain have to match the increased MTU size. Different L2 domains are not a problem because router should be able to adapt/fragment packets entering L2 domain with smaller MTU size.
Neighbor interface it is an interface of Dlink switch, where i configured L3 interface with MTU 1700.
If you tried ping from RB itself, you’ll have to increase MTU on bridge as well .. bridge is L2/L3 interface which handles pings in this case.
I dont think that it works that way, because i have an Ui version with same mtu on bridge and it works well. And i cant change MTU on bridge for some reason. L2 MTU 1598 and L3 MTU 1500. Will try to change that 951G model on something else for test.
I can change bridge MTU with set [ find name=bridge ] mtu=1580 on my RB951G just fine:
[admin@RB951G janus] /interface bridge> print
Flags: X - disabled, R - running
0 R name="bridge" mtu=1500 actual-mtu=1500 l2mtu=1598 arp=enabled
arp-timeout=auto mac-address=E6:8D:8C:C6:11:C4 protocol-mode=none
fast-forward=no igmp-snooping=no auto-mac=no
admin-mac=E6:8D:8C:C6:11:C4 ageing-time=5m vlan-filtering=no
dhcp-snooping=no
[admin@RB951G janus] /interface bridge> set [ find name=bridge ] mtu=1580
[admin@RB951G janus] /interface bridge> print
Flags: X - disabled, R - running
0 R name="bridge" mtu=1580 actual-mtu=1580 l2mtu=1598 arp=enabled
arp-timeout=auto mac-address=E6:8D:8C:C6:11:C4 protocol-mode=none
fast-forward=no igmp-snooping=no auto-mac=no
admin-mac=E6:8D:8C:C6:11:C4 ageing-time=5m vlan-filtering=no
dhcp-snooping=no
Ok, i set mtu 1598(like maximum bridge L2 mtu). but still i cant ping with packets heavier than 1534 bytes.
Is there any setting on the DGS management interface regarding MTU? Specs say it supports jumbo frames up to 13kB in size, but that’s probably switch chip capability. Management interface might have some other settings.
Personally I’d try to leave out the DGS when testing if larger frames work … so try to ping the L3 peer of your host. As I wrote before, next-hop router of RB should be able to deal with desired MTU size so it should be able to reply to pings.
I have a chain Cisco 4900M - Dlink DGS3620-28SC - Mikrotik RB951G. And i can ping from Cisco to Dlink’s address(from RB subnet) with 1700 and df-bit, so i don’t think that it is Dlink problem. Is it possible that in that RB installed old chip that doesn’t support some functions? CPU Ar9344 v3.33
That’s a real option I’m afraid. Or a bug in software. I suggest you ask support@mikrotik.com to comment your problem.
Upgrade device to latest ROS, restart then also upgrade the “Firmware” in System—>Routerboard—>Upgrade, then restart again.
Check in same place if “Current Firmware” is the same as ROS version upgraded to. The reason I am suggesting this is that there were a couple of big changes / improvements in version 3.41 IIRC and seems you are still on firmware v3.33
Leave MTU as is.
If you need more MTU try to use VPLS tunnels.
Omg, sorry! Problem was in…fiber media converter. With wrong mode. Thanks you for your advices.