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Amazon Elastic Compute Cloud
User Guide for Linux Instances

Network Maximum Transmission Unit (MTU) for Your EC2 Instance

The maximum transmission unit (MTU) of a network connection is the size, in bytes, of the largest permissible packet that can be passed over the connection. The larger the MTU of a connection, the more data that can be passed in a single packet. Ethernet packets consist of the frame, or the actual data you are sending, and the network overhead information that surrounds it.

Ethernet frames can come in different formats, and the most common format is the standard Ethernet v2 frame format. It supports 1500 MTU, which is the largest Ethernet packet size supported over most of the Internet. The maximum supported MTU for an instance depends on its instance type. All Amazon EC2 instance types support 1500 MTU, and many current instance sizes support 9001 MTU, or jumbo frames.

Jumbo Frames (9001 MTU)

Jumbo frames allow more than 1500 bytes of data by increasing the payload size per packet, and thus increasing the percentage of the packet that is not packet overhead. Fewer packets are needed to send the same amount of usable data. However, outside of a given AWS region (EC2-Classic), a single VPC, or a VPC peering connection, you will experience a maximum path of 1500 MTU. VPN connections and traffic sent over an Internet gateway are limited to 1500 MTU. If packets are over 1500 bytes, they are fragmented, or they are dropped if the Don't Fragment flag is set in the IP header.

Jumbo frames should be used with caution for Internet-bound traffic or any traffic that leaves a VPC. Packets are fragmented by intermediate systems, which slows down this traffic. To use jumbo frames inside a VPC and not slow traffic that's bound for outside the VPC, you can configure the MTU size by route, or use multiple elastic network interfaces with different MTU sizes and different routes.

For instances that are collocated inside a placement group, jumbo frames help to achieve the maximum network throughput possible, and they are recommended in this case. For more information, see Placement Groups.

The following instances support jumbo frames:

  • Compute optimized: C3, C4, CC2

  • General purpose: M3, M4, T2

  • Accelerated computing: CG1, G2, P2

  • Memory optimized: CR1, R3, R4, X1

  • Storage optimized: D2, HI1, HS1, I2, I3

Path MTU Discovery

Path MTU Discovery is used to determine the path MTU between two devices. The path MTU is the maximum packet size that's supported on the path between the originating host and the receiving host. If a host sends a packet that's larger than the MTU of the receiving host or that's larger than the MTU of a device along the path, the receiving host or device returns the following ICMP message: Destination Unreachable: Fragmentation Needed and Don't Fragment was Set (Type 3, Code 4). This instructs the original host to adjust the MTU until the packet can be transmitted.

By default, security groups do not allow any inbound ICMP traffic. To ensure that your instance can receive this message and the packet does not get dropped, you must add a Custom ICMP Rule with the Destination Unreachable protocol to the inbound security group rules for your instance. For more information, see the Adding Rules to a Security Group and API and Command Overview sections in the Amazon EC2 Security Groups topic.

Important

Modifying your instance's security group to allow path MTU discovery does not guarantee that jumbo frames will not be dropped by some routers. An Internet gateway in your VPC will forward packets up to 1500 bytes only. 1500 MTU packets are recommended for Internet traffic.

Check the Path MTU Between Two Hosts

You can check the path MTU between two hosts using the tracepath command, which is part of the iputils package that is available by default on many Linux distributions, including Amazon Linux.

To check path MTU with tracepath

  • Use the following command to check the path MTU between your Amazon EC2 instance and another host. You can use a DNS name or an IP address as the destination; this example checks the path MTU between an EC2 instance and amazon.com.

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    [ec2-user ~]$ tracepath amazon.com 1?: [LOCALHOST] pmtu 9001 1: ip-172-31-16-1.us-west-1.compute.internal (172.31.16.1) 0.187ms pmtu 1500 1: no reply 2: no reply 3: no reply 4: 100.64.16.241 (100.64.16.241) 0.574ms 5: 72.21.222.221 (72.21.222.221) 84.447ms asymm 21 6: 205.251.229.97 (205.251.229.97) 79.970ms asymm 19 7: 72.21.222.194 (72.21.222.194) 96.546ms asymm 16 8: 72.21.222.239 (72.21.222.239) 79.244ms asymm 15 9: 205.251.225.73 (205.251.225.73) 91.867ms asymm 16 ... 31: no reply Too many hops: pmtu 1500 Resume: pmtu 1500
    In this example, the path MTU is 1500.

Check and Set the MTU on your Amazon EC2 Instance

Some AMIs are configured to use jumbo frames on instance that support them, and others are configured to use standard frame sizes. You may want to use jumbo frames for network traffic within your VPC or you may want to use standard frames for Internet traffic. Whatever your use case, we recommend verifying that your instance will behave the way you expect it to. You can use the procedures in this section to check your network interface's MTU setting and modify it if needed.

To check the MTU setting on a Linux instance

  • If your instance uses a Linux operating system, you can review the MTU value with the ip command. Run the following command to determine the current MTU value:

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    [ec2-user ~]$ ip link show eth0 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000 link/ether 02:90:c0:b7:9e:d1 brd ff:ff:ff:ff:ff:ff

    In the above example, the mtu 9001 in the output indicates that this instance uses jumbo frames.

To set the MTU value on a Linux instance

  1. If your instance uses a Linux operating system, you can set the MTU value with the ip command. Run the following command to set the desired MTU value. This procedure sets the MTU to 1500, but it is the same for 9001.

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    [ec2-user ~]$ sudo ip link set dev eth0 mtu 1500

  2. (Optional) To persist your network MTU setting after a reboot, modify the following configuration files, based on your operating system type. This procedure covers Amazon Linux and Ubuntu; for other distributions, consult their specific documentation.

    • For Amazon Linux, add the following lines to your /etc/dhcp/dhclient-eth0.conf file.

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      interface "eth0" { supersede interface-mtu 1500; }

    • For Ubuntu, add the following line to /etc/network/interfaces.d/eth0.cfg.

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      post-up /sbin/ifconfig eth0 mtu 1500

  3. (Optional) Reboot your instance and verify that the MTU setting is correct.

Troubleshooting

If you experience connectivity issues between your EC2 instance and an Amazon Redshift cluster when using jumbo frames, see Queries Appear to Hang in the Amazon Redshift Cluster Management Guide