MTU
The Maximum Transmission Unit (MTU) is the largest number of bytes in an individual datagram or packet in a communications link. In the context of Ethernet, it is the largest possible frame size in a Protocol Data Unit (PDU) on the Layer 2 (data) of the OSI model. Ethernet networks use a MTU size of 1500 bytes, resulting in a frame size of 1518 bytes (14 for Ethernet header, 4 for Frame Check Sequence (FCS)). The 1500 byte MTU size is near universal on access networks.
Each layer of encapsulation adds an additional protocol header overhead which reduces the amount of data that can fit within a single 1500-byte packet.
| Protocol | Overhead |
|---|---|
| GRE (IP Protocol 47) (RFC 2784) | 24 bytes (20 byte IPv4 header, 4 byte GRE header) |
| 6in4 encapsulation (IP Protocol 41, RFC 4213) | 20 bytes |
| 4in6 encapsulation (e.g. DS-Lite RFC 6333) | 40 bytes |
| Any Additional IPv4 Header | 20 bytes |
| IPsec encryption performed by the DMVPN, ESP-AES-256 and ESP-SHA-HMAC | 73 bytes |
| MPLS | 4 bytes per label in the stack |
| IEEE 802.1Q tag | 4 bytes (Q-in-Q would add 8 bytes) |
| VXLAN | 50 bytes |
| OTV | 42 bytes |
| LISP | 36 bytes for IPv4 and 56 bytes for IPv6 encapsulation |
| NVGRE | 42 bytes |
| STT | 54 bytes |
IPv4 routers fragment packets on behalf of the source node if the packet size exceeds the link's MTU unless the Do Not Fragment (DF) bit is set to 1. Packets that have DF set to 1 and exceeds the MTU will cause the router to reply back to the source with a ICMP 'Packet Too large' message. Path MTU Discovery (PMTUD) leverages this behavior to discover the maximum MTU that can be sent to a destination.
IPv6 routers must support MTU sizes of 1280 bytes or larger and will not fragment IPv6 packets on behalf of the source.
You can use ping to send a ICMP packet of a particular size with the DF bit set to detect the maximum MTU size over a link.
| Header text | Header text |
|---|---|
| Windows | ping 10.1.1.1 -l 1500 -f
|
| Linux | ping -s 1500 -M do 10.1.1.1
|
| MacOS | ping -D -s 1500 10.1.1.1
|
Note: You might need to adjust the size value if ping does not account for the ICMP header (8 bytes) or the IPv4 header (20 bytes).
Setting MTU
On Linux, the MTU size can be set using ip link set.
# ip link set enp3s0 mtu 6128
# ip a | grep mtu
2: enp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 6128 qdisc fq_codel state UP group default qlen 1000
When setting the MTU, ensure that your network device is capable of handling the desired MTU. Exceeding this limit will result in an error. Eg.
# ip link set enp3s0 mtu 9000
Error: mtu greater than device maximum.
To have the MTU value persistent, see the relevant sections below.
Red Hat
Red Hat systems have the MTU size set in the /etc/sysconfig/network-scripts network config file. For example,
# echo "MTU=6128" >> /etc/sysconfig/network-scripts/ifcfg-enp3s0
Systemd
When using Systemd, you can either set it through systemd-networkd or a custom systemd unit.
Look at /usr/lib/systemd/network/ and create a corresponding override file in /etc/systemd/network/ with MTUBytes set to the desired MTU size.
To create a custom systemd unit, do something similar to:
# cat /etc/systemd/system/setmtu@.service
[Unit]
Description=Set mtu on device
Before=network.target
[Service]
Type=oneshot
EnvironmentFile=/etc/conf.d/setmtu
ExecStart=/usr/bin/ip link set dev %i up mtu ${%i}
[Install]
WantedBy=multi-user.target
# cat /etc/conf.d/setmtu
eth0=4000