MII monitor vs. ARP monitor

The MII monitor is driver-dependent. It monitors the links from the device to the nearest connected switch. If the failure occurs beyond the nearest connected switch, it cannot be detected by MII monitor. However, the ARP monitor is based on the communication to the target hosts designated by their IP addresses. Even if the link is beyond the nearest connected switch, the APR monitor can detect it.

Figure 1 shows a network typology in which Host 1 is connected to two independent switches. Both of the switches are connected to an external network. If link 2 is broken, both MII and ARP monitors will be able to detect it, since the link is within the nearest connected switch. If link 3 is broken, only ARP monitor can detect it.

The MII monitor is appropriate for bonding when the network communication is within a LAN. The ARP IP target-based monitor is suitable for bonding that is expected to communicate to an outside network.