Hi, can anyone please tell me the difference between (MTU) & (Actual MTU), and also what is the difference between (L2 MTU) & (Max L2 MTU) in an interface, thanks.

My 5 cents ... I might be wrong.

MTU is property of an interface and is upwards limited by capacity of underlying layers. Nowadays it's mostly limited by configuration setting though. It is a property of particular transport layer (as defined by OSI), without stating explicitly one can only assume, in this particular case it is MTU for L3 (IP layer) including it's overhead.

actual MTU is actual value and depends on many factors, including physical link properties, potential protocol overhead and possible negotiation with remote device (in case of PtP connections). It is equal or less than MTU (described in previous paragraph)

L2 MTU is similar, but for ethernet layer. Modern ethernet chips support large frames that allow for either jumbo-frames (in your case up to 4k bytes) or fancy ethernet functionality such as VLAN, MPLS and other (which add some protocol overhead but to maintain standard L3 MTU of 1500 bytes, L2 frames need to get larger).

max L2 MTU is FYI and max value you can set AS L2 MTU when configuring particular port.

Example: PPPoE over VLAN: negotiated L3 MTU for PPPoE connection can be 1480 bytes. That connection is piggy-backing VLAN with actual MTU 1500 bytes, configured MTU same 1500 bytes and L2 MTU 1594 bytes. That VLAN is using ethernet port with MTU 1500 bytes, but with L2 MTU being 1598 bytes. So you can see that physical device (ethernet port) has L2 MTU large enough to accomodate many "L2.5" protocols with their overhead and still come out with standard L3 MTU of 1500 bytes. PPPoE is slightly different story as it does depend on remote end (and possibly a few moxes in between) which might not offer larger L2 MTU, hence resulting MTU 1480 bytes.

In Mikrotik world MTU = layer3 MTU and generally shouldn't ever exceed 1500
L2 MTU = L3 MTU + all overheads (i.e. VLAN, MPLS, VPLS etc) but does NOT include the mandatory 14 byte ethernet header nor the Frame Check Sequence of 4 bytes. So it is acceptable to have an MTU of 1500 and L2 MTU of 1500 on a physical interface with no vlans etc

Generally you want to set L3 MTU to 1500 and leave L2MTU alone, or set it to the highest possible because by default that's just giving you headroom and won't cause any incompatibilities

In Mikrotik world MTU = layer3 MTU and generally shouldn't ever exceed 1500

I'm late in this thread but i post for those who will get here searching for MTU in Mikrotik.

Jumbo frames would be more than 1500 bytes. For example, we have GlusterFS storage nodes hooked to a 10 Gbps switch with jumbo frame enabled @ 9000 bytes. In high throughput network, this will improve throughput.

We have L2MTU set to 9092 and MTU to 9000.

https://wiki.mikrotik.com/wiki/Manual:M ... uterBoards

In Mikrotik world MTU = layer3 MTU and generally shouldn't ever exceed 1500

I'm late in this thread but i post for those who will get here searching for MTU in Mikrotik.

Jumbo frames would be more than 1500 bytes. For example, we have GlusterFS storage nodes hooked to a 10 Gbps switch with jumbo frame enabled @ 9000 bytes. In high throughput network, this will improve throughput.

We have L2MTU set to 9092 and MTU to 9000.

Code: Select all

/interface ethernet
set sfp-sfpplus8 l2mtu=9092 mtu=9000

https://wiki.mikrotik.com/wiki/Manual:M ... uterBoards

9000MTU for only 10Gbe interface or all network should change for 9000MTU ?

9000MTU for only 10Gbe interface or all network should change for 9000MTU ?

MTU should be the same size on the whole ethernet broadcast domain. It doesn't work if one host is configured to use large MTU while another host on same ethernet broadcast domain uses normal MTU. This only works if there's a router between such hosts, which performs fragmentation on packets from host with large MTU towards host with smaller MTU.

Only device which can perform fragmentation (and is thus capable of receiving large packets in the first place) can cooperate in Path MTU discovery process.