Configuring the MTU for an Ethernet Interface
MTU is the largest packet of data that can be transmitted on a network, expressed in bytes. MTU is determined by data link layer protocols, and MTU values vary with networks. A proper MTU is a prerequisite for normal communication between network devices.
Context
The size of packets is limited at the network layer. Whenever receiving an IP packet, the IP layer determines the next-hop interface for the packet and obtains the MTU configured on the interface. Then, the IP layer compares the MTU with the packet length. If the packet length is longer than the MTU, the IP layer fragments the packet into smaller packets, which are shorter than or equal to the MTU.
If unfragmentation is configured, some packets may be discarded during data transmission at the IP layer. To ensure that jumbo packets are not dropped during transmission, set an MTU on an interface to fragment these packets into smaller ones.
- After changing the MTU on a specified interface, you need to restart the interface to validate the newly-configured MTU by running the shutdown and undo shutdown commands in sequence.
- If the Ethernet interface has sub-interfaces, you need to run the undo shutdown command 15 seconds after running the shutdown command.
If the size of packets is much greater than the configured MTU value, the packets are broken into a great number of fragments. The packets may be discarded by quality of service (QoS) queues.
If the configured MTU value is too large, packets may be transmitted at a low speed.
- After changing the MTU of an Ethernet interface, also change the MTU of the peer Ethernet interface to ensure that the MTUs of both interfaces are the same. If the MTUs are different, services may be interrupted.
Perform the following steps on each Router:
Follow-up Procedure
The length of a QoS queue is limited. If the size of packets is much greater than the configured MTU value, the packets are broken into a great number of fragments. The packets may be discarded by the QoS queue. To address this problem, you can increase the length of the QoS queue. The queue scheduling mechanism First In First Out (FIFO) is used on an interface by default. You can change the FIFO queue length. For detailed configuration of a QoS queue, see HUAWEI NE40E-M2 Series Universal Service Router Configuration Guide - QoS.
