ARP flow in Cisco SD-Access

ARP flow in Cisco SD-access

Earlier we talked about the DHCP process in SD-Access environment, now once the end client gets the IP, it wants to communicate with another client through the Fabric ( SD-Access) and it generates the ARP request to understand the path for communication.

Let us consider here there are two end clients which are now active and Client 1 needs to communicate to Client 2 through the SD-Access Fabric. 

Fig 1.1- ARP flow in Cisco SD-Access

Step 1: ARP request
Due to the fact that Client1 is in the same subnet as Client2, it generates an ARP request for Client2's IP address. As we have Edge node where this client 1 connected will not flood the arp request but will send a map-request for the IP address included in the "Target IP address" to the Control Plane node ( which can be Border node in most of the cases)

Step 2: Map Server (LISP) first request
All hosts in the fabric are mapped to IP-MAC addresses in the address-resolution table maintained by control plane nodes.  Map requests (generated from ARP requests) are looked up in this table when they reach the control plane (Border1).In case it finds a entry, it sends a map-response with the host's mac address.

Step 3: Map Server (LISP) Second request
It then builds another map-request using the mac address it got from the control-plane, this time for the mac address it just got from the control-plane. As a result, there is a second round of map-requests and map-replies.Border1 responds with the RLOC associated with the mac address that it received (which is Edge2) when it receives the map request.

ARP is now converted into a unicast message on Edge1, encapsulated with Edge2's IP and encapsulated with a VXLAN header with a VNID of the Ethernet service (for that VLAN).Edge2's Loopback0 is the destination IP.

Once Edge2 gets this, it decapsulates the packet and unicasts it to Client2. This entire process now happens in reverse when Client2 sends an ARP reply in response to the ARP request.

In case, we don't have information on step 2 as map server request, then Edge 1 will do L2 flooding through the fabric and force a silent host to respond, thus bringing it alive in the fabric. We will