Correct MTU for QinQ through EoIP

RouterOS General
1

Hi everyone!

I hope that anyone here can guide me in the right direction regarding the following scenario:

RouterBOARD 1:
ether1 has a tagged VLAN 10 “subinterface” with a /30 IP address
eoip-tunnel1 is bridged with ether2

RouterBOARD 2:
ether1 has also a tagged VLAN 10 “subinterface” with the other /30 IP address
eoip-tunnel1 is bridged with ether2

The MTU of the switch where both RouterBOARDs ether1 interfaces are connected to is 1536.


Now I want a Cisco 3550 on both sides connected to ether2 to be able to run dot1q-tunnels accross the eoip-tunnel1, which means the tagged port between the 3550s (which is the eoip-tunnel1) has additional 4bytes attached for the second VLAN tag next to the standard 4byte VLAN tag.

So please let me know which parameter (MTU or L2MTU) of which interface I have to set to which value that this scenario works without problems??


I already searched the forum but didn’t find a straigt answer :frowning:



Thanks and kind regards,
uebi

2

Read this:
http://wiki.mikrotik.com/wiki/Manual:BCP_bridging_(PPP_tunnel_bridging)
http://wiki.mikrotik.com/wiki/Maximum_Transmission_Unit_on_RouterBoards

I advice you to use BCP over L2TP with MLPPP support over single link instead of EoIP, then you don’t have to mind about the packet size through the tunnel.
Regarding the MTU size, it’s the L2MTU you should look at.
L2MTU is the maximum size of a packet in the physical interface, read the second guide above.

If you have problems, send a printout from:

/int bri export
/int vlan exp
/int eth exp

//Paul

3

Even if I don’t need MLPPP? I’m just looking for PtP and QinQ, nothing more, nothing less.


So for a QinQ packet comming from a Cisco Trunk the RoutBOARD’s interface L2MTU should be 1504?


Thanks and kr,
uebi

4

A QinQ packet have 2 VLAN tags, so the L2MTU needs to be at least 1508.

5

Well, that’s what I’m not sure about (if we are talking about the LAYER 2 FRAME):

802.1q trunking - Header: 4 bytes = 1500 + 4 + 18 = 1522 L2
QinQ - Header: 4 + 4 bytes = 1500 + 4 + 4 + 18 = 1526 L2

So L2MTU (which sounds to me like Layer 2 MTU G) should be 1504 to allow a full QinQ IP paket I suppose?!?


KR,
uebi

6

Maybe I misunderstand you,

does EoIP runs over the vlan10?
Then I agree that the ether2 L2MTU should be 1504
In your example, it’s not QinQ you’re running, since EoIP encapsulates the whole ethernet frame from ether2
I still advice you to run BCP with L2TP instead of EoIP it’s less CPU intensive.

BR,
Paul