VLAN MTU issue

RouterOS General
1

I have a routerboard with two ports bridged together. Under the bridge I add a vlan. Running OSPF on the router. If I reboot the router I start getting this error:

Discarding database description packet: too large MTU mine=1458 received=1500

If I check the MTU of the vlan it is set to 1500.

As soon as I disable the vlan and re-enable it the problem goes away. It doesn’t seem to reappear unless I reboot.

Hardware CCR-1009-8G-1S running 6.30.1

2

Hello

I have a similar problem (6.47.1, “cAP ac” with IP 100.64.5.197 and NAT):

11:08:48.574235 IP x.x.x.6.https > 100.64.5.197.34552: Flags [.], seq 1:1461, ack 518, win 30016, length 1460
11:08:48.574836 IP 100.64.5.197 > x.x.x.6: ICMP 100.64.5.197 unreachable - need to frag (mtu 1458), length 556




/interface vlan
add interface=ether1 name=ether1.608 vlan-id=608




/interface vlan pr
Flags: X - disabled, R - running
 #   NAME                                       MTU ARP             VLAN-ID INTERFACE
 0 R ether1.608                                1500 enabled             608 ether1

"/interface vlan set 0 mtu=1542 " seems to fix the problem, but it looks strange.

3

"/interface vlan set 0 mtu=1542 " seems to fix the problem

But it can probably cause traffic problems from the router itself.

4

Even more interesting. The router had in the initial configuration

> interface br pr
Flags: X - disabled, R - running
 0 R ;;; defconf
     name="bridge" mtu=auto actual-mtu=1458 l2mtu=1594 ...

and internal and used ether1.001 included to it:

/interface bridge port
add bridge=bridge comment=defconf interface=ether2
add bridge=bridge comment=defconf interface=wlan1
add bridge=bridge comment=defconf interface=wlan2
add bridge=bridge interface=ether1.001

“interface br set 0 mtu=1500” solve the problem also (without increasing MTU on an external ether1. 608).