These 2 links come up in a google search and just introduce more confusion
https://mum.mikrotik.com//presentations ... 279781.pdf - page 13
https://mum.mikrotik.com/presentations/US13/kirnak.pdf - page 19
The first shows MTU requirements for MPLS
The first column makes sense. You want an MTU of 1500 at Layer3, MPLS introduces 8 bytes of overhead, add the ethernet frame and you end up with 1522, perfect
What doesn't make sense are the other 2 columns
If you have an MPLS MTU of 1526 why is the L2 MTU also 1526? Wouldn't it be 1540? (1526 + 14 for ethernet = 1540)
In addition to that, aren't the L2 figures 4 bytes short? Because they aren't taking into account the Frame Check Sequence, as shown in the second link
The red lines are wrong aren't they? Shouldn't the L2 MTU in the top section be 1518? and the bottom would then be 1530
Official documentation about ethernet standards is as follows
So is this a MikroTik thing where the 'L2 MTU' field in routers/switches doesn't actually include the FCS and thus will always be 4 bytes smaller?The frame size of a standard Ethernet frame (defined by RFC 894) is the sum of the Ethernet header (14 bytes), the payload (IP packet, usually 1,500 bytes), and the Frame Check Sequence (FCS) field (4 bytes)
In essence, if I want to send a full 1500 byte IP packet across a MikroTik router with the bare minimum MTU value, do I set MTU as 1500, and then L2 MTU as 1514 (but it actually sends 1518). Or would it be MTU 1500 and L2 MTU 1518?