[FAQ]Why Does MAC Address Triggered ARP Entry Update Need to Be Configured When Highlighted

Created: Dec 29, 2015 14:10:50Latest reply: Dec 29, 2015 14:15:07 1756 1 0 0

On the Ethernet, MAC address entries are used to guide Layer 2 data forwarding. The ARP entries that define the mapping between IP addresses and MAC addresses guide communication between devices on different network segments.

In the scenario where devices of a VRRP group connect to servers, the outbound interface in a MAC address entry is updated by packets, whereas the outbound interface in an ARP entry is updated after the aging time is reached. In this case, the outbound interfaces in the MAC address entry and ARP entry may be different. To address this issue, run the mac-address update arp command to enable the MAC address triggered ARP entry update function. In the following figure, SwitchA and SwitchB function as gateways of the server and have VRRP enabled to enhance reliability. VRRP packets are transmitted on the directly connected link between the two switches. When the server sends packets, only one network interface is selected to forward packets. When a network fault or traffic exception is detected, another network interface is used.

  • SwitchA functions as the master device and the server uses Port1 to send packets. SwitchA learns the ARP entry and MAC entry on Port1, and SwitchB learns the server MAC address on Port2.
  • When the server detects that Port1 is faulty, the server uses Port2 to forward service packets. SwitchA learns the server MAC address on Port2. If the server does not send an ARP Request packet to SwitchA, SwitchA still learns the ARP entry on Port1. In this case, packets sent from SwitchA to the server are forwarded through Port1 until the ARP entry is aged out.

To solve the problem, configure MAC address triggered ARP entry update. This function enables the device to update the corresponding ARP entry when the outbound interface in a MAC address entry changes.

MAC address triggered ARP entry update is disabled by default. You can run the mac-address update arp command to enable MAC address triggered ARP entry update.

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who_knows     Created Dec 29, 2015 14:15:07 Helpful(0) Helpful(0)

This command(mac-address update arp) takes effect only for dynamic ARP entries. Static ARP entries are not updated when the corresponding MAC address entries change.

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