ipv6 mtu

Function

The ipv6 mtu command sets the MTU on the interface for sending IPv6 packets.

The undo ipv6 mtu command restores the default MTU value.

Format

ipv6 mtu mtu

undo ipv6 mtu

Parameters

Parameter	Description	Value
mtu	Specifies the MTU value.	USG6510E/6510E-POE/6530E, USG6515E, USG6635E/6655E, USG6525E, USG6615E/6625E and USG6635E/6655E: For BDIF interfaces: The value is an integer, in bytes. The value ranges from 1280 to 1500. The default value is 1450 bytes. For other interfaces: The value is an integer, in bytes. The value ranges from 1280 to 1500. The default value is 1500 bytes. USG6550E/6560E/6580E, USG6555E/6565E/6575E-B/6585E/6605E-B, USG6610E/6620E, USG6630E/6650E, USG6680E and USG6712E/6716E: For BDIF interfaces: The value is an integer, in bytes. The value ranges from 1280 to 9500. The default value is 1450 bytes. For other interfaces: The value is an integer, in bytes. The value ranges from 1280 to 9500. The default value is 1500 bytes.

Views

Layer-3 Ethernet interface view, Layer-3 Ethernet sub-interface view, BDIF interface view, Eth-Trunk interface view, Eth-Trunk sub-interface view, VLANIF interface view, Tunnel interface view, Dialer interface view, Virtual-Template interface view

Default Level

2: Configuration level

Usage Guidelines

Usage Scenario

The size of data frames is limited at the network layer. Any time the IP layer receives an IP packet, it checks to which local interface the packet needs to be sent and obtains the MTU of the interface. The IP layer then compares the MTU with the packet length. If the packet length is longer than the MTU, the IP layer disassembles the packet to fragments, each not longer than the MTU. If forcible unfragmentation is configured, some packets may be discarded during data transmission at the IP layer. Therefore, large packets must be forcibly fragmented to prevent packet loss on the network. The mtu command is used to set the size of a fragment.

A correct MTU value is the prerequisite for normal and efficient communication between devices.

If the MTU is too small and packets are large, the packets will be discarded when they are forwarded by hardware or will be divided into excessive fragments when they are forwarded by the CPU, affecting data transmission.
If the MTU is too large, it may exceed the maximum value set for the receive end or an intermediate device. In this case, packets will be fragmented or even discarded, increasing the network transmission load or affecting normal data transmission.

You are advised to use the default MTU. If the size of transmitted packets or packet receiving device changes, you can adjust the MTU based on the actual network environment.

Precautions

USG6510E/6510E-POE/6530E, USG6515E, USG6635E/6655E, USG6525E, USG6615E/6625E and USG6635E/6655E: A packet larger than 1636 bytes will be discarded by the inbound interface. USG6550E/6560E/6580E, USG6555E/6565E/6575E-B/6585E/6605E-B, USG6610E/6620E, USG6630E/6650E, USG6680E and USG6712E/6716E: A packet larger than 9518 bytes will be discarded by the inbound interface.

This command can be configured on an interface and its sub-interface. The MTU values of the interface and its sub-interface do not affect each other.

The MTU configured on an Eth-Trunk interface has impacts on its member interfaces. The MTU cannot be configured for Eth-Trunk member interfaces, and the MTU of the Eth-Trunk interface takes effect on its member interfaces.

For a Dialer interface whose PPP link is Up, the changed MTU cannot take effect. Instead, you can change the MTU for this interface only when its PPP link is Down.

You are advised to set the MTU of the Tunnel interface less than the source interface MTU deducing the packet header length.

Example

# Set the MTU on GigabitEthernet 0/0/1 for sending IPv6 packets to 1280 bytes.

<sysname> system-view
[sysname] interface GigabitEthernet 0/0/1
[sysname-GigabitEthernet0/0/1] ipv6 enable
[sysname-GigabitEthernet0/0/1] ipv6 mtu 1280