Dynamic Learning of ARP Entries with Multicast MAC Addresses

In Figure 1, the Switch is connected to the NLB cluster through a Layer 2 switch (LSW). The NLB cluster is working in multicast mode, and the cluster IP address is mapped to a multicast MAC address. After the Switch 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 Switch does not learn the ARP entry if it finds that the source MAC address in the received ARP packet is a multicast MAC address. As a result, the Switch cannot communicate with the NLB cluster.

Figure 1 Network diagram of dynamic learning of multicast MAC addresses

With dynamic learning of multicast MAC addresses enabled, the Switch learns and generates ARP entries for ARP Reply packets whose source MAC addresses are multicast addresses.