Configuring Packet Relay for DHCP Servers

In general, routers do not forward broadcast packets from one subnet to another.  However, there are cases in which the broadcast packets need to be passed onto other subnets. One typical example is DHCP (Dynamic Host Configuration Protocol) in which a new computer needs to acquire an available IP address by broadcasting the requests to DHCP server. DHCP protocols work seamlessly if both DHCP client and DHCP server are located in the same broadcast domain. However,  that does not work across broadcast domains without supporting features such as DHCP relay.  That is, when the DHCP server is not in the same subnet as the clients, use DHCP relay for broadcasting traffic.

Packet Relay is the router functionality that helps forward broadcast packets between broadcast domains. In Netvisor ONE  architect, the packet relay functionality is implemented as a service running within a particular vRouter.

You can configure a vRouter to relay DHCP requests from local clients to a centralized DHCP server. Since the initial DHCP request arrives from a client that typically does not have an IP address, the client must find the DHCP server using a Layer 2 broadcast.

The DHCP server needs information before the server can allocate an IP address to the client. It must know the subnet and the MAC address of the client. The DHCP server needs the subnet information to ensure that the IP address that the client receives can work on the client’s subnet. The MAC address is necessary so that the DHCP server can find any information that is unique to the client.

When you configure DHCP relay on a vRouter, the vRouter converts the local broadcast packet from the client to a unicast packet and forwards it to the server.

Since the DHCP client does not have an IP address when it sends the DHCP request packet, the client uses the IP address, 0.0.0.0, as the source IP address and the general broadcast address 255.255.255.255 for the destination.

The vRouter replaces the source address with the IP address assigned to the interface where the request is received, and replaces the destination IP address with the address you specify in the vRouter packet-relay command.

To configure packet-relay for a DHCP server with the IP address 172.16.21.34 and vRouter interface (on the network where the client is connected to) - eth11.100, use the following syntax:

CLI (network-admin@Leaf1) > vrouter-packet-relay add vrouter-name vrouter-dhcp forward-proto dhcp forward-ip 172.16.21.34 nic eth11.100

Once you add the configuration, you cannot modify it. If you make a mistake or want to add a new configuration, you must use the vrouter-packet-relay-remove command.