ipv6 pathmtu

Function

The ipv6 pathmtu command sets a static path maximum transmission unit (PMTU) for a specified destination IPv6 address.

The undo ipv6 pathmtu command deletes the static PMTU set for a specified destination IPv6 address.

By default, no static PMTU is set for a specified destination IPv6 address.

Format

ipv6 pathmtu ipv6-address [ vpn-instance vpn-instance-name ] [ path-mtu ]

undo ipv6 pathmtu ipv6-address [ vpn-instance vpn-instance-name ]

Parameters

Parameter	Description	Value
ipv6-address	Specifies the IPv6 address for which a PMTU is to be set.	The address is a 32-bit hexadecimal number, in the format of X:X:X:X:X:X:X:X. The address can be a unicast address only.
vpn-instance vpn-instance-name	Specifies the name of an IPv6 VPN instance for which a PMTU is to be set.	The value is a string of 1 to 31.
path-mtu	Specifies the path MTU, that is, the maximum size of IPv6 packets allowed to be sent along the path.	The value is an integer ranging from 1280 to 10000, in bytes. The default value is 1500.

Parameter

Description

Value

ipv6-address

Specifies the IPv6 address for which a PMTU is to be set.

The address is a 32-bit hexadecimal number, in the format of X:X:X:X:X:X:X:X. The address can be a unicast address only.

vpn-instance vpn-instance-name

Specifies the name of an IPv6 VPN instance for which a PMTU is to be set.

The value is a string of 1 to 31.

path-mtu

Specifies the path MTU, that is, the maximum size of IPv6 packets allowed to be sent along the path.

The value is an integer ranging from 1280 to 10000, in bytes. The default value is 1500.

Views

System view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
ip-stack	write

Usage Guidelines

Usage Scenario

A Path Maximum Transmission Unit (PMTU) is used to determine the proper size of packets to be transmitted along the path from a source to a destination. Commonly, a device fragments and forwards packets based on the dynamic PMTU learning function of the system, trying not to fragment packets during transmission. In this manner, loads on intermediate Routers are reduced and effective network resource use.

In some special cases, however, to protect devices on the network and avoid the large-sized packet attacks, you can run the ipv6 pathmtu command to set a static PMTU for the specified destination IPv6 address to control the maximum size of packets that can be transmitted between the source and the destination.

Configuration Impact

On the path along which packets are transmitted, a node discards the received packets if its MTU is smaller than the PMTU of the received packets. Therefore, in most cases, dynamic PMTU learning is recommended unless there are security vulnerabilities on the network. You do not need to run the ipv6 pathmtu command to set a static PMTU, that is, use the default PMTU.

Precautions

The priorities of the static PMTU, dynamic PMTU, and default PMTU of the system are in a descending order.

Example

# Set the PMTU for the address 2001:db8::1 in the IPv6 VPN instance to 1600 bytes.

<HUAWEI> system-view
[~HUAWEI] ip vpn-instance vpn6
[*HUAWEI-vpn-instance-vpn6] ipv6-family
[*HUAWEI-vpn-instance-vpn6-af-ipv6] quit
[*HUAWEI-vpn-instance-vpn6] quit
[*HUAWEI] ipv6 pathmtu 2001:db8::1 vpn-instance vpn6 1600

# Set the PMTU for the specified IPv6 address to 1300 bytes.

<HUAWEI> system-view
[~HUAWEI] ipv6 pathmtu 2001:db8::12 1300