Cisco ACI Unknown Unicast: Hardware Proxy vs Flooding Mode

Before we start, lets once again make sure we fully understand what Bridge Domain is. The bridge domain can be compared to a giant distributed switch. Cisco ACI preserves the Layer 2 forwarding semantics even if the traffic is routed on the fabric. The TTL is not decremented for Layer 2 traffic, and the MAC addresses of the source and destination endpoints are preserved.

When you configure the Bridge Domain in the ACI, you need to decide what you want to do with the ARP packets, and what you want to do with the Unknown L2 Unicast. You can basically:

  • Enable ARP Flooding, or not.
  • Choose between the two L2 Unknown Unicast modes: Flood and Hardware Proxy.




Hardware Proxy

By default, Layer 2 unknown unicast traffic is sent to the spine proxy. This behaviour is controlled by the hardware proxy option associated with a bridge domain: if the destination is not known, send the packet to the spine proxy; if the spine proxy also does not know the address, discard the packet (default mode).

The advantage of the hardware proxy mode is that no flooding occurs in the fabric. The potential disadvantage is that the fabric has to learn all the endpoint addresses.

With Cisco ACI, however, this is not a concern for virtual and physical servers that are part of the fabric: the database is built for scalability to millions of endpoints. However, if the fabric had to learn all the IP addresses coming from the Internet, it would clearly not scale.


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.

The advantage of disabling hardware-based proxy and using flooding for unknown hosts and ARP is that the fabric does not need to learn millions of source IP addresses coming from a given port.

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