arp broadcast enable

Function

The arp broadcast enable command enables the Address Resolution Protocol (ARP) broadcast function on a VLAN tag termination sub-interface.

The undo arp broadcast enable command disables the ARP broadcast function on a VLAN tag termination sub-interface.

By default, the ARP broadcast function is disabled on VLAN tag termination sub-interfaces.

Format

arp broadcast enable

undo arp broadcast enable

Parameters

None

Views

100ge sub-interface view, 10GE sub-interface view, 200GE sub-interface view, 25GE sub-interface view, 400GE sub-interface view, 40GE sub-interface view, 50GE sub-interface view, Eth-Trunk sub-interface view, GE sub-interface view, Global VE sub-interface view, PW-VE sub-interface view, VE sub-interface view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
ethernet	write

Usage Guidelines

Usage Scenario

When IP packets reach a VLAN tag termination sub-interface which does not have ARP entries configured, the sub-interface directly discards the IP packets if the ARP broadcast function is not enabled on the sub-interface. To enable the sub-interface to forward the IP packets, run the arp broadcast enable command.

Prerequisites

An encapsulation type has been configured on the VLAN tag termination sub-interface using the encapsulation command.

The encapsulation dot1q-termination command is replaced by the control-vid dot1q-termination command.

Configuration Impact

If an IP packet is sent to a VLAN tag termination sub-interface without a corresponding ARP entry, the following situations are available:

If the access device can automatically send ARP packets, the IP packet can be forwarded without the need to configure the ARP broadcast function on the VLAN tag termination sub-interface.
If the access device cannot automatically send ARP packets,
The device discards the IP packet if the arp broadcast enable command is not configured on the VLAN tag termination sub-interface. In this case, the route with the VLAN tag termination sub-interface as the outbound interface is considered a black hole route.
The device sends tagged ARP broadcast packets through the VLAN tag termination sub-interface if the arp broadcast enable command is used on the VLAN tag termination sub-interface.

Precautions

When you enable or disable the ARP broadcast function on a VLAN tag termination sub-interface, the routing status of the sub-interface goes Down and then Up. This may result in route flapping on the entire network, affecting service running.

After the ARP broadcast function is enabled using the arp broadcast enable command, if a service packet is sent from a termination sub-interface but does not have an ARP entry, an ARP request is copied and sent in all VLANs of the sub-interface. If a large number of VLANs are configured, the number of ARP requests to be copied and sent is large, which may bring a heavy burden on the peer devices that receive the ARP requests. Consequently, the peer devices may encounter exceptions, such as high CPU usage and broadcast suppression. The local device may also fail to immediately send ARP requests as it is busy in processing packet copying, leading to ARP learning failures. To prevent this issue, you are advised to reduce the number of VLANs configured on sub-interfaces.

To address the problem that the device fails to send ARP packets in time due to replication of lots of packets, you can run the broadcast-copy fast enable command to enable the hardware replication function. However, this command configuration consumes multicast resources and the sending of lots of ARP packets causes heavy load on the peer device. Therefore, you are advised to contact Huawei engineers to determine whether to enable the hardware replication function.

Example

# Enable the ARP broadcast function on GE 0/1/1.1.

<HUAWEI> system-view
[~HUAWEI] interface GigabitEthernet 0/1/1.1
[*HUAWEI-GigabitEthernet0/1/1.1] encapsulation dot1q-termination
[*HUAWEI-GigabitEthernet0/1/1.1] arp broadcast enable