L2 & L3 Maximum Transmissible Unit (MTU)

When talking about L2 MTU (maximum transmissible unit), we are talking about how all frames have to be within that limit. Wheras with L3 MTU, we are talking about the entire L3 packet.

• For example if the L2 MTU is 1500B then a frame with >1500B will be dropped.
• For example if the L3 MTU is 1500B then a packet with >1500B will be fragmented.

The default L2 & L3 MTU MTU is 1500B for most Cisco platforms, but can be increased to usually around >= 9000B.

Two Types of MTU

L2 MTU focuses on frames coming into a L2 device, determining if it hits the L2 MTU threshold, then drops it if it does.

L3 MTU focuses on packets coming into a L2 device, determining if it hits the L3 MTU threshold, then fragments it if it does.

Use Cases

L2 Example

The ethernet spec allows for a 14B header, a 1500B payload, then 4B for the CRC header. Keep in mind a bonus 4B for 802.1Q if added.

Lets take a frame like this:

Header	802.1Q	Payload	CRC Trailer
14 Bytes	4 Bytes	1500 Bytes	4 Bytes

—> L2 MTU applies ONLY to the PAYLOAD <—

In this situation, all these frames would be allowed, but if the L2 MTU was shorted to 1000 Bytes, then the frame would be dropped.

L3 Example

Take this example standard packet for us to use as a starting point:

Layer	Component	Size (Bytes)
Layer 2	Ethernet Header	14
Layer 3	IPv4 Header	20
Layer 4	TCP/UDP Header	20
Layer 7	App Data (Payload)	1460
Layer 2	CRC Trailer	4
Total on wire	--->	1518 bytes (without VLAN)

—> L3 MTU applies ONLY to the IP Packet (IP + TCP/UDP + data) <—

The default L3 MTU of 1500 bytes fits within the L2 MTU of 1500 bytes because:

• L2 payload = entire L3 packet
• So 1500 bytes of IP packet (including IP + TCP/UDP + data) fits exactly within Ethernet’s 1500-byte payload field

Configurations

Switch Command Reference

(config)# system mtu [bytes]

# show system mtu

Router Command Reference

interface Ethernet1/1
  mtu [bytes]
  ip mtu [bytes]

ping [ipaddress] size [bytes] {df-bit}

Note that L3 MTU (ip mtu) will default to matching the L2 MTU configuration, and will not show up in the show interface command.