Configuring Dynamic ARP

Hosts and routers can learn dynamic ARP entries by default. However, you can adjust dynamic ARP aging parameters or optimize the ARP entry update policy.

Usage Scenario

ARP aging parameters include the aging time, number of ARP aging probes, and ARP aging probe interval. Proper setting of these aging parameters can improve network reliability:

Aging time: When the aging time of a dynamic ARP entry expires, a device sends an ARP probe packet (ARP Request message) from the outbound interface recorded in the dynamic ARP entry, and starts counting the number of ARP probes.
Number of ARP aging probes: Before deleting an aged dynamic ARP entry, a device sends ARP probe packets to the IP address recorded in the ARP entry at specified intervals. If the configured number of ARP aging probes is exceeded but the ARP entry has not been updated, the device will delete the ARP entry.
ARP aging probe interval: It is the interval at which probe packets are sent.
1. If the aging time of dynamic ARP entries is set too short, for example, 1 minute, the device will be busy updating dynamic ARP entries. This consumes a lot of system resources and affects the processing of other services.
2. Length of time before the deletion of a dynamic ARP entry = Number of ARP aging probes x Probe interval
  
  Setting a long probe interval is not recommended, because a long interval will delay the deletion of an aged dynamic ARP entry according to the formula.

After the Layer 2 topology detection function is enabled, the aging time of all ARP entries corresponding to the VLAN to which a Layer 2 interface belongs is set to 0 when the status of the Layer 2 interface changes from Down to Up. Then the device resends ARP probe packets to update all ARP entries.

After the device on a ring network is disabled from responding to TC packets, the device will not age or delete ARP entries after receiving TC packets, reducing resource consumptions caused by frequent ARP entry updates and minimizing adverse impacts on user services.

Pre-configuration Tasks

Before configuring dynamic ARP, complete the following tasks:

Configure physical parameters for interfaces to ensure that the physical status of the interfaces is Up.
Configure link layer protocol parameters for interfaces to ensure that the link layer protocol status of the interfaces is Up.

Procedure

Adjust dynamic ARP aging parameters.
1. Run system-view
  The system view is displayed.
2. Run interface interface-type interface-number
  The interface view is displayed.
3. Run arp expire-time expire-time
  The aging time is set for dynamic ARP entries.
  
  In specific scenarios, after MAC entries are aged, the VLANIF interface fails to obtain the Layer 2 outbound interface and broadcast a unicast packet in the VLAN. To ensure service security, you can run the arp expire-time expire-time vlanif command to set the aging time for dynamic ARP entries on all VLANIF interfaces to a value smaller than the aging time for MAC entries. This prevents unicast packets from being broadcast due to Layer 2 outbound interface mismatch.
4. Run arp detect-times detect-times
  The number of ARP aging probes is set.
5. Run arp detect-interval detect-interval
  The ARP aging probe transmission interval is set.
6. Run arp detect-mode unicast
  The interface is configured to unicast ARP aging probe packets.
7. Run commit
  The configuration is committed.
Enable Layer 2 topology detection.
1. Run system-view
  The system view is displayed.
2. Run l2-topology detect enable
  Layer 2 topology detection is enabled.
Disable the device from responding to TC packets.
1. Run system-view
  The system view is displayed.
2. Run arp topology-change disable
  The device is disabled from responding to TC packets.

Verifying the Configuration of Dynamic ARP

After configuring dynamic ARP, verify the configuration.

Run the display arp all command to check the ARP entries on the main control boards and all interface boards.
Run the display arp interface interface-name command to check the ARP entries on a specified interface.
Run the display arp slot slot-id command to check the ARP entries on a board in a specified slot.
Run the display arp vpn-instance vpn-instance-name slot slot-id [ dynamic | static ] command to check the ARP entries of a specified VPN instance.