# Multicast MAC

In case it is not feasible to use a switch that works in unicast mode with clusters, a shared multicast MAC can be defined for the cluster nodes.

Most switches support this mode, however, not all switches in the same virtual LAN (VLAN) need to be configured. By default, most switches send packets with a multicast MAC address to all ports connected to the same VLAN. If the size of the VLAN is small, this type of flooding is acceptable. However, with larger VLANs performance problems can occur as the device needs to send each packet to each port connected to the same VLAN. In some switches, it’s possible to prevent this type of flooding by statically restricting multicast traffic with a given MAC address to some predefined ports only.

Note: Some networking devices discard ARP replies specifying a multicast MAC. In this case, static ARP entries must be used.

The following illustration presents an example where a common multicast MAC is configured for all cluster nodes. For instance, if a switch is not able to send packets with the same unicast MAC to multiple ports, this type of configuration might be used. Each node has also a unique unicast MAC address mapped to the corresponding IP addresses defined at the NDIs.

Figure: CVI with multicast MAC

Interface (external) | Node 1 | Node 2 | Node 3
--- | --- | --- | ---
CVI IP Address | 203.0.113.254 | 203.0.113.254 | 203.0.113.254
CVI Unicast MAC | 09:08:08:08:08:08 | 09:08:08:08:08:08 | 09:08:08:08:08:08
NDI IP Address | 203.0.113.21 | 203.0.113.22 | 203.0.113.23
NDI Unicast MAC | 04:08:08:08:08:08 | 06:08:08:08:08:08 | 08:08:08:08:08:08