DHCP process works in Cisco SD-Access

DHCP works in SD-Access

Hope you understand the DHCP process in the traditional network where the end client or host send the request as broadcast request to get the IP. The process is the same but there is something more which is happing to work DHCP in the overlay Fabric network.

Lets start how DHCP works for endpoint to get the new IP address from the DHCP server via SD-Access Fabric network. 

Endpoint sends DHCP broadcast request to edge switch with MAC address as source.

When the edge switch see a DHCP request coming from the endpoint, it uses DHCP snooping and adds Option 82 fields including the VXLAN VNI or instance id, along with the RLOC address as remote ID, before encapsulating it into a unicast packet with the SVI Anycast address as the source and the IP address of the DHCP server as the destination.

As we know that DHCP server is outside the fabric network and this packet needs to reach outside the fabric, the packet is routed in overlay via fabric Border to fusion router.

Fig 1.1- DHCP request in SD-Access

ow when the DHCP server replied back to request of Anycast SVI address and is received to Border Node via Fusion , Border Node device see option 82 field which contains the Edge RLOC and instance ID. Border node forwards that reply directly to that particular Edge switch.

Once fabric edge receives the reply, it de-encapsulates the packet and then forwards the new DHCP reply to endpoint.