There are multiple scenarios where DHCP is a requirement in the data center. VDI is one major use-case among others. VDI clients request DHCP IP addresses from local data center DHCP servers. A large percentage of data centers leverage Windows Servers for DHCP. With Cisco ACI, an additional DHCP Option 82 is required to be processed in order to properly assign DHCP IP Addresses. Legacy Windows Operating System DHCP server such as Server 2003, 2008 and 2012 have caveats around supporting Cisco ACI fabric networks. New operating systems such as Server 2016 or Server 2019 are needed in order to fully support ACI network topologies.
Option 82 is a DHCP Relay Agent Information Option. It is used to provide additional contextual information in the DHCP request such as Physical Interface and VLAN ID of where the Client resides on the DHCP Relay Proxy Gateway, TEP address of the DHCP Relay Proxy Gateway (Leaf Switch), VRF Name/VPN ID, Server ID and Link Selection information. All of these Sub-options of Option 82 have critical information needed to properly assign IP addresses to the correct location on the Cisco ACI fabric. Keep in mind ACI has tenants, VRFs, Bridge Domains which can all have overlapping constructs and are not as straight forward as traditional networks. We have new concepts like pervasive gateways where the default gateway and MAC can be on all of the leaf switches. These all have to be factored in and considered for DHCP in VXLAN networks.