MAC Address-Triggered ARP Entry Update

On an Ethernet network, a host sends and receives Ethernet data frames based on MAC addresses. The Address Resolution Protocol (ARP) maps IP addresses to MAC addresses. When two devices on different network segments communicate with each other, they need to map IP addresses to MAC addresses and outbound interfaces according to ARP entries.

Generally, the outbound interfaces in the matching MAC address entries and ARP entries are consistent. As shown in Figure 2-6, the outbound interface in both the MAC address entry and ARP entry is 10GE1/0/1. The interface is then changed. At T2, after a packet is received from the peer device, the outbound interface in the MAC address entry is immediately changed to 10GE1/0/2. However, the outbound interface in the ARP entry is still 10GE1/0/1. At T3, the aging time of the ARP entry expires, and the outbound interface in the ARP entry is changed to 10GE1/0/2 through ARP aging probe. Between T2 and T3, the outbound interface in the ARP entry is unavailable, interrupting communication between devices on different network segments.

Figure 2-6 MAC address-triggered ARP entry update is not enabled

MAC address-triggered ARP entry update enables a device to update the outbound interface in an ARP entry immediately after the outbound interface in the corresponding MAC address entry changes. As shown in Figure 2-7, MAC address-triggered ARP entry update is enabled. At T2, after the outbound interface in the MAC address entry is changed to 10GE1/0/2, the outbound interface in the ARP entry is immediately changed to 10GE1/0/2. This function prevents communication interruption between T2 and T3 due to the incorrect outbound interface in the ARP entry.

Figure 2-7 MAC address-triggered ARP entry update is enabled

In data center virtualization scenarios, when the location of a virtual machine (VM) changes, user traffic on the network may be interrupted if the VM cannot send gratuitous ARP messages promptly to update ARP entries on the gateway. In this case, the device relearns ARP entries by exchanging ARP messages only after ARP entries on the gateway age.