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Configuring Routers, Switches, and Interfaces as DHCP and BOOTP Relay Agents

You can configure the router, switch, or interface to act as a Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) relay agent. This means that a locally attached host can issue a DHCP or BOOTP request as a broadcast message. If the router, switch, or interface sees this broadcast message, it relays the message to a specified DHCP or BOOTP server.

You should configure the router, switch, or interface to be a DHCP and BOOTP relay agent if you have locally attached hosts and a distant DHCP or BOOTP server. For MX Series routers connected via IRB, see the note below to prevent BOOTP reply packets from being dropped.

To configure the router or switch to act as a DHCP and BOOTP relay agent, include the bootp statement at the [edit forwarding-options helpers] hierarchy level:

To set the description of the BOOTP service, DHCP service, or interface, include the description statement.

To set a logical interface or a group of logical interfaces with a specific DHCP relay or BOOTP configuration, include the interface statement.

To set the routing instance of the server to forward, include the routing-instance statement. You can include as many routing instances as necessary in the same statement.

To stop packets from being forwarded on a logical interface, a group of logical interfaces, or the router or switch, include the no-listen statement.

To set the maximum allowed number in the hops field of the BOOTP header, include the maximum-hop-count statement. Headers that have a larger number in the hops field are not forwarded. If you omit the maximum-hop-count statement, the default value is four hops.

To set the minimum allowed number of seconds in the secs field of the BOOTP header, include the minimum-wait-time statement. Headers that have a smaller number in the secs field are not forwarded. The default value for the minimum wait time is zero (0).

To set the IP address that specify the DHCP or BOOTP server for the router, switch, or interface, include the server statement. You can include multiple server statements.

To set an IP time-to-live (TTL) value for DHCP response packets sent to a DHCP client, include the client-response-ttl statement.

To use the DHCP relay agent option in relayed BOOTP/DHCP messages, include the relay-agent-option statement. This option is primarily useful for enabling DHCP forwarding between different VRF routing instances. This option is documented in RFC 3046, DHCP Relay Agent Information Option.

You can also configure an individual logical interface to be a DHCP and BOOTP relay agent if you have locally attached hosts and a remote DHCP or BOOTP server connected to one of the router's or switch’s interfaces. For more information, see the Junos OS Administration Library for Routing Devices.

The following example demonstrates a BOOTP relay agent configuration.

Best Practice:

To use bootp helper on a MX Series router (MX80, MX240, MX480 and MX960) connected via IRB, you may need to take steps to ensure that DHCP discover packets (the bootp reply) are sent to clients and received as expected. Otherwise, bootp replies may be dropped because the DHCP client is clearing the broadcast bit in the discover packet, or because the DHCP server is stripping option-82 flags from the offer.

This happens when the IRB interface is a layer 3 (logical) interface associated with a bridge domain that has multiple layer 2 (physical) interfaces associated with it. In such cases, if the offer from the DHCP server is unicast and doesn’t include an ingress interface identifying the physical interface on which the discovery packet was received, the MX router won’t be able to determine an interface for sending out offers.

  1. Enable broadcast on the IRB interface to flood discovery frames from all physical interfaces in the bridge domain. For example,


  2. Enable relay-agent-option on the bootp helper. For example,

  3. Configure the IRB interface connected to the DHCP server so it echoes option-82 flags back to the router. This will ensure that the option-82 string, which identifies the interface used by the router, is preserved.