Dynamic Learning of ARP Entries with Multicast MAC Addresses
By default, a device does not learn ARP entries when receiving the ARP packets in which the source MAC addresses are multicast MAC addresses. If the device is enabled to learn the ARP entries containing multicast MAC addresses, the device can generate ARP entries after receiving ARP packets carrying multicast MAC addresses as source MAC addresses.
As shown in Figure 2-6, the Router is connected to the NLB cluster through the Layer 2 switch LSW. The NLB cluster works in multicast mode. That is, the cluster IP address is mapped to a multicast MAC address. After the Router sends an ARP Request packet requesting the cluster MAC address, the NLB servers send an ARP Reply packet with the cluster MAC address as the source MAC address. By default, the Router does not learn the ARP entry if it determines that the source MAC address in the received ARP packet is a multicast MAC address. As a result, the Router cannot communicate with the NLB cluster.
After dynamic learning of multicast MAC addresses is enabled, the Router can generate ARP entries after receiving ARP Reply packets carrying multicast MAC addresses as source MAC addresses.