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DHCP Relay Agent in EVPN-MPLS Network

 

Dynamic Host Configuration Protocol (DHCP) is a protocol that allows a DHCP server to dynamically allocate IP addresses to DHCP clients. The DHCP relay agent forward DHCP messages between DHCP clients and DHCP servers when they are on different networks. Junos supports configuring the DHCP relay agent on the spine devices in a centrally bridging overlay on an EVPN-MPLS network.

Figure 1 shows a centrally-routed bridging EVPN-MPLS network with a virtual machine on a datacenter server multihomed to leaf 1 and leaf 2. The leaf devices are connected to spine devices that function as DHCP relays and forwards the DHCP messages to the DHCP server.

Figure 1: DHCP Relay Agent on Centrally-Routed Bridging Overlay
 DHCP Relay Agent on Centrally-Routed
Bridging Overlay

The following sequence describes the interaction between the DHCP client, DHCP relay agent, and DHCP server.

  1. The DHCP client (VM) initiates a request by sending a DHCP discovery message to find a DHCP server and obtain a lease for an IP address. DHCPv6 clients sends DHCPv6 solicit messages.

  2. Leaf 1 broadcasts the new locally-learned MAC address and DHCP discovery (or DHCPv6 solict) message to the other devices in the EVPN core.

  3. Spine 1 forwards the DHCP discovery (or DHCPv6 solicit) message to DHCP server.

  4. Upon receipt of the DHCP discovery (or DHCPv6 solicit) message, the DHCP server issues a DHCP offer message (or DHCPv6 advertisement) message with the IP address back to Spine 1.

  5. Spine 1 forwards the DHCP offer (or DHCPv6 advertisement) message to Leaf 1 where the message is sent back to the client.

Subsequent DHCP request (or DHCPv6 request) and DHCP acknowledgment (or DHCPv6 reply) messages are similarly forwarded. For DHCP renew and DHCP release messages, the DHCP client sends unicast messages to renew or release their IP address. The DHCP V6 clients sends multicast messages to renew or release their IP address.

Benefits of DHCP Relay Agent

DHCP allows you to manage IP addresses and other network configurations easily. The DHCP relay on the spine devices of a centrally routed bridging overlay allows you to simplify and centralized the routing of the DHCP messages.

Configuring DHCP Relay Agents in EVPN-MPLS

To configure the DHCP relay agent on the Junos OS device, include the dhcp-relay statement at Uthe [edit forwarding-options] hierarchy level for either DHCP or DHCPv6 services.

To configure the DHCP server, create a server group with at least one DHCP server IP addresses on the DHCP relay agent and apply the server-group as a member of an active-server-group.

To configure a group of DHCP server addresses and define them as an active server group:

  1. Specify the name of the server group.
  2. Add the IP addresses of the DHCP servers belonging to the group.
  3. Define the server group as an active server group.

The following configuration snippet shows a sample DHCP relay configuration.

Note

DHCP relay agent on EVPN-MPLS does not support active leasequery (ALQ).