What is DHCP Relay?

Dynamic Host Configuration Protocol (DHCP) servers provide configuration parameters to DHCP clients. DHCP supplies network settings, including the host IP address, the default gateway, and a DNS server. When DHCP hosts do not reside on the same IP network or subnet, a DHCP relay agent can transfer DHCP messages between them. The DHCP relay agent operates as the interface between DHCP clients and the server. It listens for client requests and adds configuration data, such as the client's link information, which is needed by the server to allocate the address for the client. When the DHCP server responds, the DHCP relay agent forwards the reply back to the DHCP client.

In ACI this process is enable per Bridge Domain, and then ACI will automatically push the configuration parameters across the entire fabric. This is different from today's network where network admins are required to configure dhcp relay commands on every interface and every device. This is another advatange of ACI when it comes to automating network functions because your fabric could be as small as 2 leafs or as large as 200+ leafs and ACI will automate this process.

It is important to understad how ACI operates when it comes to flooding and DHCP relay, while ACI fabric-wide flooding is disabled by default, flooding within a bridge domain is enabled by default. Because flooding within a bridge domain is enabled by default, hosts can connect to DHCP servers within the same EPG. However, when the DHCP server is in a different EPG or BD than the clients, DHCP Relay is required.

In this section, students will be creating the DHCP construct for the Virtual Machines to be able to obtain their DHCP addresses. It is important to understand that the DHCP policy that we will be creating will be using the aci_p21_bd_mgmt that we created during the previous step.

Step 1 - Navigate to DHCP Relay Labels in the Tenants Tab under Networking - Brigde Domains

Navigate to Bridge Domains to create the DHCP Relay Labels by clicking:

1. Click on Tenant
2. Search for aci_p21_tenant
3. Expand Networking
4. Expand Bridge Domains
5. Expand aci_p21_bd_mgmt
6. Right Click on DHCP Relay Labels
7. Click Create DHCP Relay Label

Step 2 - Create the DHCP Relay Label

This step will allow the ACI admin to create the DHCP relay label by providing the scope. The scope or the relay owner can have the following values:

• Infra: Only DHCP relay policies created in the Fabric > Access Policies tab will be displayed. This configuration should be used for a policy that is intended to be used fabric wide.
• Tenant: Only DHCP relay policies created in the tenant will be displayed.

This lab will leverage the Tenant scope in order for each student to create their own DHCP Relay Label.

1. Scope: Tenant
2. Name: Pull down Menu
3. Click on Create DHCP Relay Policy

Step 3 - Create the DHCP Relay Policy

The Relay Policy is where the DHCP server information is going to be defined. We need to provide the name of the policy and in the information of the DHCP server.

1. Name: aci_p21_dhcp
2. Then click the plus sign (+) button to add Providers

Step 4 - Create the DHCP Provider

This lab leverages an external DHCP server in order to provide DHCP functions to the hosts that are attached to the ACI Fabric. Therefore we need to modify the defaul values in ACI from Applicatin EPG to L3 External Network.

1. EPG Type: L3 External Network
2. L3 External Network common
3. L3 Out: k8s
4. External Network: k8s-epg
5. DHCP Server Address: 10.0.226.7
6. Click OK

7. It is important to click Submit after configuring the DHCP Server in order for the changes to take effect.

Step 5 - DHCP Relay Label created

With this the DHCP relay label should be created and you can verify the value under the BD definition in the fabric.

Before proceeding to the next section, verify your work.