MTU · Topic 2 of 3

L2MTU and Jumbo Frames

Understand the difference between MTU and L2MTU on RouterOS, why jumbo frames need support on every hop, and how to raise the sizes safely and test them.

  • 5 min
In a nutshell
  • The standard Ethernet MTU is 1500 bytes.
  • L2MTU is the largest frame an interface can process, and it must be larger than the MTU.
  • /interface print detail shows both mtu and l2mtu.

What it is

MTU limits the packet, L2MTU limits the whole frame, and jumbo frames are frames larger than the standard size.

The standard Ethernet MTU is 1500 bytes. That is the largest Layer 3 packet an interface sends without fragmenting it.

IP packetMTU limitplus headers and tagsEthernet frameL2MTU limitsent onWireto next hop

L2MTU is the largest frame the interface can handle. It must be at least as big as the MTU plus the Layer 2 overhead, and the overhead grows when you add VLAN tags.

How it works

Both values are shown per interface, and both must be high enough on every device along the path.

Reading the values

The /interface listing shows both numbers.

/interface print detail
/interface ethernet print

mtu is a setting you change. l2mtu is mostly a property of the hardware and driver. On many devices you cannot change it, and on some you can.

The L2MTU differs between models and even between ports of the same device. A small router may allow a little above the standard size, and a CRS switch chip supports a much larger value.

MTU versus L2MTU

PropertyCountsTypical use
mtuThe Layer 3 packetSet to what the IP layer should send
l2mtuThe full Layer 2 frameMust allow the MTU plus tags and headers

If you raise the MTU of an interface above what its L2MTU can carry, the interface rejects the value or frames get dropped. Check the L2MTU first.

VLAN and other overhead

Every added header shrinks the room for payload, or demands a bigger L2MTU.

  • A VLAN tag adds four bytes.
  • A stacked second tag, as in QinQ, adds four more bytes.
  • Tunnels such as EoIP and VPLS add their own headers.
  • PPPoE adds a small header that lowers the usable MTU.

A VLAN interface is created on top of a parent interface. Its L2MTU is typically four bytes smaller than the parent, because the parent has to carry the tag.

Jumbo frames

Jumbo frames carry more payload per frame, so the same data needs fewer frames.

The result is less header overhead and fewer interrupts for the CPU of a server, which helps bulk transfers such as storage traffic. A value near 9000 is a common choice, but both ends must agree on it. There is no single official jumbo size, so use a value that every device on the path supports.

Jumbo frames must be enabled end to end:

  1. Servers or hosts at both ends.
  2. Every switch port in between.
  3. Every bridge, VLAN interface and router interface on the path.
/interface ethernet set ether2 mtu=9000
/interface bridge set bridge1 mtu=9000
/interface vlan set vlan10 mtu=9000

Only the devices that actually handle the larger frame need the larger value. Within one broadcast domain, all hosts that talk to each other with jumbo frames should use the same MTU.

Exam tip

Set the jumbo MTU on the host and on the interface that faces it, then verify with a do-not-fragment ping of the full size.

Verifying

Send a ping that fills a full jumbo frame.

/ping 10.0.0.2 size=9000 do-not-fragment

In RouterOS the ping size is the whole IP packet, so size=9000 fills a 9000 byte MTU. A ping tool that counts only the data, such as the one on most hosts, needs 8972 bytes of data, because the IP and ICMP headers use 28 bytes. If the ping fails and a smaller one passes, a hop along the path has a smaller limit.

What to remember for the exam

Seven facts cover most questions on this topic.

  • The standard Ethernet MTU is 1500 bytes.
  • L2MTU is the largest frame an interface can process, and it must be larger than the MTU.
  • /interface print detail shows both mtu and l2mtu.
  • A VLAN tag adds four bytes, and a second tag adds four more.
  • Jumbo frames are larger than the standard size, and a value near 9000 is common.
  • Jumbo frames must be supported on every device and port along the path.
  • A RouterOS ping with size=9000 and do-not-fragment tests a 9000 byte path. A host ping that counts only data needs 8972.
Watch out

Hosts on the same segment with different MTU values can exchange small packets but fail on large ones, which is hard to diagnose.

Common mistakes

These four mistakes appear often.

Setting MTU above L2MTU. The interface cannot carry the larger frame, so the value is refused or traffic is lost.

Enabling jumbo frames on only some hops. One standard size switch port in the path is enough to drop jumbo frames.

Ignoring tag overhead. A tagged jumbo frame needs room for the tag as well.

Mixing up ping size conventions. RouterOS counts the whole IP packet, while most host ping tools count only the data and add 28 bytes of header.

Check yourself

Work through these three questions.

  • What is the difference between MTU and L2MTU?

    MTU limits the Layer 3 packet size, and L2MTU limits the whole Layer 2 frame an interface can handle.

  • Why must jumbo frames be configured end to end?

    Any hop with a smaller limit drops the larger frames, so every device and port on the path must support them.

  • Which ping tests a 9000 byte path?

    /ping with size=9000 and do-not-fragment on RouterOS, where the size includes the headers. A host ping that counts only data needs 8972 bytes.

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