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 OS  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,, as the source IP address and the general broadcast address 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 and vRouter interface (on the network where the client is connected to) - eth11.100, use the following syntax:


CLI (network-admin@switch) > vrouter-packet-relay add vrouter-name vrouter-dhcp forward-proto dhcp forward-ip 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.

Configuring DHCP Packet Relay over VXLAN

NetVisor OS version 6.1.0 supports DHCP packet relay configuration over VXLAN. To demonstrate this functionality, consider the topology below:

Figure 5-1 - DHCP Relay Configuration over VXLAN

The nodes switch1 and switch2 belong to two separate subnets and have VXLAN connectivity between them. For more information, see the Configuring VXLAN chapter. The DHCP servers 1 and 2 are connected to switch1 while the DHCP client is connected to switch2. The DHCP servers and the DHCP client belong to the VRF vrf1.

From NetVisor OS version 6.1.1, DHCP relays are VRF-aware. To create a VRF, use the command:

CLI (network-admin@switch1) > vrf-create name vrf1 scope fabric

Follow the steps below to configure DHCP relay between the DHCP servers and the client.

Create a VLAN for the DHCP client and associate a VXLAN with the VLAN:

CLI (network-admin@switch2) > vlan-create id 101 scope fabric vxlan 100101

Create VLANs for the DHCP servers and associate the VLANs with VXLANs:

CLI (network-admin@switch1) > vlan-create id 103 scope fabric ports 25 vxlan 100102

CLI (network-admin@switch1) > vlan-create id 104 scope fabric ports 35 vxlan 100103

Create a vRouter and vRouter interface for VLAN 101:

CLI (network-admin@switch2) > vrouter-create name vr1 vnet vnet1 router-type hardware

CLI (network-admin@switch2) > vrouter-interface-add vrouter-name vr1 ip vlan 101 vlan-type public if data if-nat-realm internal vrf vrf1

Note: You also need to configure VRRP interfaces if the DHCP client is attached to a cluster.

View the configuration using the vrouter-interface-show command:

CLI (network-admin@switch1) > vrouter-interface-show vlan 101 format vrouter-name,nic,ip,mac,vlan,vlan-type,nic-state

vrouter-name nic      ip               mac               vlan vlan-type nic-state 

------------ -------- ---------------- ----------------- ---- --------- ---------

vr1          eth2.102 66:0e:94:79:34:fe 101  public    up      

Associate the vRouter to the VRF by using the command:

CLI (network-admin@switch1) > vrouter-vrf-add vrouter-name vr1 vrf vrf1 bgp-as 65100 router-id bgp-redistribute connected

Use the vrouter-vrf-show command to view the vRouter VRF configuration:

CLI (network-admin@switch1) > vrouter-vrf-show format switch,vrouter-name,vrf,hw-vrid,bgp-as,router-id,bgp-redistribute

switch  vrouter-name vrf      hw-vrid bgp-as router-id bgp-redistribute

------- ------------ -------- ------- ------ --------- ----------------

switch1 vr1          vrf1     1       65100 connected       

Configure DHCP packet relay for both DHCP servers:

CLI (network-admin@switch2) > vrouter-packet-relay-add vrouter-name vr1 forward-ip forward-proto dhcp nic eth2.102

CLI (network-admin@switch2) > vrouter-packet-relay-add vrouter-name vr1 forward-ip forward-proto dhcp nic eth2.102

Here, forward-ip is the IP address of the DHCP server, forward-proto is the protocol supported by packet relay which is DHCP, and nic is the ingress interface of the vRouter on which DHCP broadcast packets are expected. The VRF ID is derived from the NIC value provided to the vrouter-packet-relay-add command.

View the packet relay configuration by using the command:

CLI (network-admin@switch2) > vrouter-packet-relay-show

vrouter-name forward-proto forward-ip   nic      vrf

------------ ------------- ------------ -------- ----

vr1          dhcp eth2.102 vrf1

vr1          dhcp eth2.102 vrf1


  • If the DHCP client is connected through a vLAG to a cluster pair, you must configure vRouters, vRouter interfaces, and DHCP packet relay for both the cluster switches.
  • If you configure the DHCP relay and filter on different nodes, the uplink port (from the DHCP filter node to the relay) and the downlink port (from the DHCP filter node to the server interface) must be configured as trusted ports. For more information, see Support for DHCP Snooping section of the Configuring Network Security chapter.
  • You cannot specify VRRP VIP NICs in the vrouter-packet-relay command. You can specify VRRP primary NICs instead.