mtu

Function

The mtu command sets the maximum transmission unit (MTU) value for the management VLANIF and CAPWAP on an AP.

The undo mtu command restores the default MTU value for the management VLANIF and CAPWAP on an AP.

By default, the MTU value of the management VLANIF and CAPWAP on an AP is 1500 bytes.

Format

mtu mtu-value

undo mtu

Parameters

Parameter	Description	Value
mtu-value	Specifies the maximum size of packets sent and received on the management VLANIF and CAPWAP.	The value is an integer that ranges from 1500 to 1700, in bytes.

Views

AP system profile view

Default Level

2: Configuration level

Usage Guidelines

Usage Scenario

The MTU determines the maximum number of bytes in IP packets each time a sender can send. The MTU of an IP packet refers to the number of bytes from the IP header of the packet to the data.

The MTU in the AP system profile view is the maximum size of packets sent and received on the management VLANIF and CAPWAP of an AP.

The size of data frames is limited at the network layer. Any time the IP layer receives an IP packet to be sent, it checks to which local interface the packet needs to be sent 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 jumbo packets are not dropped during transmission, you need to configure forcible fragmentation. In this case, you can run the mtu command to set the size of a fragment.

Therefore, a proper MTU is a prerequisite for normal communication on a network.

If the configured MTU is excessively small and the packet size is larger, packets are discarded when being forwarded through the forwarding chip; packets are broken into a great number of fragments when being forwarded through the CPU, affecting proper data transmission.
If the size of packets exceeds the MTU supported by a transit node or a receiver, the transit node or receiver fragments the packets or even discards them, aggravating the network transmission load.

The default MTU is recommended. When the size of packets to be transmitted or the device that receives packets changes, you can change the MTU based on the actual network.

Precautions

DHCP packets cannot be fragmented. When the MTU value set using the mtu command is smaller than the DHCP packet length, DHCP packets cannot be forwarded. Therefore, set a larger MTU value.
If the MTU value is smaller than the DHCP packet length, the AP may be disconnected. In this case, restart the AP.
When the MTU is too small and the DF bit is set to 1, packets cannot be fragmented. In this case, use the forced fragmentation function.

Example

# Set the MTU value of the management VLANIF and CAPWAP on an AP to 1700 bytes.

<HUAWEI> system-view
[HUAWEI] wlan 
[HUAWEI-wlan-view] ap-system-profile name ap-system1
[HUAWEI-wlan-ap-system-prof-ap-system1] mtu 1700