Dynamic ARP allows devices to dynamically learn and update the mapping between IP and MAC addresses using ARP messages. You do not need to manually configure the mapping.
Dynamic ARP uses the dynamic ARP aging mechanism.
The dynamic ARP aging mechanism enables the ARP entries that exceed the aging time to be automatically deleted. This mechanism helps reduce storage space of ARP tables and speed up ARP table queries.
Table 1 describes concepts related to the dynamic ARP aging mechanism.
Concept |
Description |
Usage Scenario |
|---|---|---|
Aging probe mode |
Before a dynamic ARP entry on a device is aged, the device sends ARP aging probe messages to the other devices on the same network segment. An ARP aging probe message can be a unicast or broadcast message. By default, a device broadcasts ARP aging probe messages. |
|
Aging time |
A dynamic ARP entry has a life cycle. If a dynamic ARP entry is not updated before its life cycle ends, this dynamic ARP entry is deleted from the ARP table. The life cycle is called aging time. |
Two interconnected devices can learn the mapping between their IP and MAC addresses using ARP and can save the mapping in their ARP tables. Then, the two devices can communicate by using the ARP entries. When the peer device becomes faulty, or the network adapter of the peer device is replaced but the local device does not receive any status change information about the peer device, the local device continues sending IP datagrams to the peer device. As a result, network traffic is interrupted because the ARP table of the local device is not promptly updated. To reduce the risk of network traffic interruption, an aging timer can be set for each ARP entry. After the aging timer of a dynamic ARP entry expires, the entry is automatically deleted. |
Number of aging probe attempts |
Before a dynamic ARP entry is aged, a device sends ARP aging probe messages to the peer device. If the device does not receive an ARP reply message after the number of aging probe attempts reaches a specified number, the dynamic ARP entry is aged. |
The ARP aging timer can help reduce the risk of network traffic interruptions that occur because an ARP table is not updated quickly enough, but cannot eliminate problems due to delays. Specifically, if the dynamic ARP entry aging time is N seconds, the local device can detect the status change of the peer device after N seconds. During the N seconds, the ARP table of the local device is not updated. If the number of aging probe attempts is specified, the local device can obtain the status change information about the peer device and update its ARP table. |
Creating and updating dynamic ARP entries
Aging dynamic ARP entries
After the aging timer of a dynamic ARP entry on a device expires, the device sends ARP aging probe messages to the peer device. If the device does not receive an ARP reply message after the number of aging probe attempts reaches a specified number, the dynamic ARP entry is aged.
The shutdown operation on the interface will trigger ARP entry aging deletion on the interface. The shutdown operation on the Admin-VS will trigger ARP entry aging deletion in the VSn.
This feature limits the rate of sending ARP probe messages in order to prevent too many system resources from being used during ARP probing. In high-specification scenarios, it usually takes a long time from when ARP probing starts to when ARP entry aging is complete.
Dynamic ARP has an enhanced Layer 2 topology probe function. This function enables a device to set the aging time to 0 for all ARP entries corresponding to a VLAN to which a Layer 2 interface belongs when the Layer 2 interface becomes Up. The device then resends ARP probe messages to update all ARP entries.
If a non-Huawei device that connects to a Huawei device receives an ARP aging probe message with the destination MAC address as the broadcast address and the ARP table of the non-Huawei device contains the mapping between the IP address and MAC address of the Huawei device, the non-Huawei device does not respond to the broadcast ARP aging probe message. Therefore, the Huawei device considers the link to the non-Huawei device Down and deletes the mapping between the IP address and MAC address of the non-Huawei device. To prevent this problem, configure Layer 2 topology change so that the Huawei device unicasts ARP aging probe messages to the non-Huawei device.
Dynamic ARP applies to a network with a complex topology, insufficient bandwidth resources, and a high real-time communication requirement.
Dynamic ARP entries are dynamically created and updated using ARP messages. They do not need to be manually maintained, greatly reducing maintenance workload.