Exam 300-620 topic 1 question 134 discussion

Flooding Mode Alternatively, you can enable flooding mode: if the destination MAC address is not known, flood in the bridge domain. By default, ARP traffic is not flooded but sent to the destination endpoint. By enabling ARP flooding, ARP traffic is also flooded. A good use case for enabling ARP flooding would be when the Default Gateway resides outside of the ACI Fabric. This non-optimal configuration will require ARP Flooding enabled on the BD. This mode of operation is equivalent to that of a regular Layer 2 switch, except that in Cisco ACI this traffic is transported in the fabric as a Layer 3 frame with all the benefits of Layer 2 multi-pathing, fast convergence, and so on. Hardware proxy and unknown unicast and ARP flooding are two opposite modes of operation. With hardware proxy disabled and without unicast and ARP flooding, Layer 2 switching would not work. This option does not have any impact on what the mapping database actually learns; the mapping database is always populated for Layer 2 entries regardless of this configuration.

For this reason, I would choose option C (L2 Unknown Unicasts are sent to the Hardware Proxy)

Not necessarily. ARP flooding is only required if the following two conditions are met: There is a silent host in a Bridge Domain There is no IP address configured for the bridge domain in the same subnet as the silent host. The reason for this is because ACI does ARP Gleaning. ARP Gleaning ONLY works if the Bridge Domain (or EPG associated with the Bridge Domain) has been assigned an IP address on the same subnet with which it can source a Gleaning ARP. The IP address assigned to the Bridge Domain does not have to be the default gateway IP – if you have a router or firewall attached that serves as a default gateway for an EPG and you DON’T want to turn on ARP flooding, assigning any IP address on that subnet to the Bridge Domain will ensure your hosts will find their default gateway.