About Proxy ARP
Proxy ARP in Cisco ACI enables endpoints within a network or subnet to communicate with other endpoints without knowing the real MAC address of the endpoints. Proxy ARP is aware of the location of the traffic destination, and offers its own MAC address as the final destination instead.
To enable Proxy ARP, intra-EPG endpoint isolation must be enabled on the EPG see the following figure for details. For more information about intra-EPG isolation and Cisco ACI, see the Cisco ACI Virtualization Guide.
Proxy ARP within the Cisco ACI fabric is different from the traditional proxy ARP. As an example of the communication process, when proxy ARP is enabled on an EPG, if an endpoint A sends an ARP request for endpoint B and if endpoint B is learned within the fabric, then endpoint A will receive a proxy ARP response from the bridge domain (BD) MAC. If endpoint A sends an ARP request for endpoint B, and if endpoint B is not learned within the ACI fabric already, then the fabric will send a proxy ARP request within the BD. Endpoint B will respond to this proxy ARP request back to the fabric. At this point, the fabric does not send a proxy ARP response to endpoint A, but endpoint B is learned within the fabric. If endpoint A sends another ARP request to endpoint B, then the fabric will send a proxy ARP response from the BD MAC.
The following example describes the proxy ARP resolution steps for communication between clients VM1 and VM2:
-
VM1 to VM2 communication is desired.
Table 1. ARP Table State Device
State
VM1
IP = * MAC = *
ACI fabric
IP = * MAC = *
VM2
IP = * MAC = *
-
VM1 sends an ARP request with a broadcast MAC address to VM2.
Table 2. ARP Table State Device
State
VM1
IP = VM2 IP; MAC = ?
ACI fabric
IP = VM1 IP; MAC = VM1 MAC
VM2
IP = * MAC = *
-
The ACI fabric floods the proxy ARP request within the bridge domain (BD).
Table 3. ARP Table State Device
State
VM1
IP = VM2 IP; MAC = ?
ACI fabric
IP = VM1 IP; MAC = VM1 MAC
VM2
IP = VM1 IP; MAC = BD MAC
-
VM2 sends an ARP response to the ACI fabric.
Table 4. ARP Table State Device
State
VM1
IP = VM2 IP; MAC = ?
ACI fabric
IP = VM1 IP; MAC = VM1 MAC
VM2
IP = VM1 IP; MAC = BD MAC
-
VM2 is learned.
Table 5. ARP Table State Device
State
VM1
IP = VM2 IP; MAC = ?
ACI fabric
IP = VM1 IP; MAC = VM1 MAC
IP = VM2 IP; MAC = VM2 MAC
VM2
IP = VM1 IP; MAC = BD MAC
-
VM1 sends an ARP request with a broadcast MAC address to VM2.
Table 6. ARP Table State Device
State
VM1
IP = VM2 IP MAC = ?
ACI fabric
IP = VM1 IP; MAC = VM1 MAC
IP = VM2 IP; MAC = VM2 MAC
VM2
IP = VM1 IP; MAC = BD MAC
-
The ACI fabric sends a proxy ARP response to VM1.
Table 7. ARP Table State Device
State
VM1
IP = VM2 IP; MAC = BD MAC
ACI fabric
IP = VM1 IP; MAC = VM1 MAC
IP = VM2 IP; MAC = VM2 MAC
VM2
IP = VM1 IP; MAC = BD MAC