Extended DHCP Relay Agent Overview

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You can configure extended DHCP relay options on the router and enable the router to function as a DHCP relay agent. A DHCP relay agent forwards DHCP request and reply packets between a DHCP client and a DHCP server. You can use DHCP relay in carrier edge applications such as video/IPTV to obtain configuration parameters, including an IP address, for your subscribers.

For more information about how to use the DHCP relay agent in a video/IPTV application, see the JUNOS Feature Guide.

Note: The extended DHCP relay agent options configured with the dhcp-relay statement are incompatible with the DHCP/BOOTP relay agent options configured with the bootp statement. As a result, you cannot enable both the extended DHCP relay agent and the DHCP/BOOTP relay agent on the router at the same time.

For information about the DHCP/BOOTP relay agent, see the JUNOS Policy Framework Configuration Guide.

To configure the extended DHCP relay agent on the router, include the dhcp-relay statement at the [edit forwarding-options] hierarchy level. See the [edit forwarding-options dhcp-relay] Hierarchy Level for the complete DHCP relay agent syntax.

You can also include the dhcp-relay statement at the following hierarchy levels:

[edit logical-systems logical-system-name forwarding-options]
[edit logical-systems logical-system-name routing-instances routing-instance-name forwarding-options]
[edit routing-instances routing-instance-name forwarding-options]

This overview covers:

Interaction Among the DHCP Relay Agent, DHCP Client, and DHCP Servers

In a typical carrier edge network configuration, the DHCP client is on the subscriber’s computer, and the DHCP relay agent is configured on the router between the DHCP client and one or more DHCP servers.

The following steps describe, at a high level, how the DHCP client, DHCP relay agent, and DHCP server interact in a configuration that includes two DHCP servers.

The DHCP client sends a discover packet to find a DHCP server in the network from which to obtain configuration parameters for the subscriber, including an IP address.
The DHCP relay agent receives the discover packet and forwards copies to each of the two DHCP servers. The DHCP relay agent then creates an entry in its internal client table to keep track of the client’s state.
In response to receiving the discover packet, each DHCP server sends an offer packet to the client. The DHCP relay agent receives the offer packets and forwards them to the DHCP client.
On receipt of the offer packets, the DHCP client selects the DHCP server from which to obtain configuration information. Typically, the client selects the server that offers the longest lease time on the IP address.
The DHCP client sends a request packet that specifies the DHCP server from which to obtain configuration information.
The DHCP relay agent receives the request packet and forwards copies to each of the two DHCP servers.
The DHCP server requested by the client sends an acknowledgement (ACK) packet that contains the client’s configuration parameters.
The DHCP relay agent receives the ACK packet and forwards it to the client.
The DHCP client receives the ACK packet and stores the configuration information.
If configured to do so, the DHCP relay agent installs a host route and Address Resolution Protocol (ARP) entry for this client.
After establishing the initial lease on the IP address, the DHCP client and the DHCP server use unicast transmission to negotiate lease renewal or release. The DHCP relay agent “snoops” on all of the packets unicast between the client and the server that pass through the router to determine when the lease for this client has expired or been released. This process is referred to as lease shadowing or passive snooping.

Access and Access-Internal Routes

The DHCP application on a video services router uses both access routes and access-internal routes to represent either the end users or the networks behind the attached router. An access route represents a network behind an attached video services router, and is set to a preference of 13. An access-internal route is a /32 route that represents a directly attached end user, and is set to a preference of 12.

To configure import and export of access routes and access-internal routes in a routing policy, include the access and access-internal keywords as match conditions at the [edit policy-options policy-statement policy-name term term-name from protocol] hierarchy level. For information, see the JUNOS Policy Framework Configuration Guide.

To display configuration information for access routes and access-internal routes, use the show route extensive, show route protocol access, and show route protocol access-internal operational commands. For command syntax and examples, see the JUNOS Routing Protocols and Policies Command Reference.

DHCP State Persistence

The extended DHCP relay agent maintains the state of active DHCP client leases in persistent storage on the router. It can recover this state if the DHCP relay agent process fails or is manually restarted, or if you manually reboot (gracefully shut down) the router. DHCP state persistence prevents the loss of active DHCP clients in either of these circumstances. If a power failure occurs or if the kernel stops operating on a single Routing Engine, however, the state of active DHCP client leases is lost.

DHCP state persistence is automatically enabled when you configure the extended DHCP relay agent on the router by including the dhcp-relay statement.

The DHCP relay agent records in persistent storage only those DHCP clients that are fully bound, which means that they currently have an active lease on an IP address from a DHCP server. DHCP clients in a renewal or rebind state are considered to be fully bound, and their state is also maintained in persistent storage. When a DHCP client lease expires or the client is released, the DHCP relay agent removes the client state from persistent storage.

Graceful Routing Engine Switchover

The extended DHCP relay agent supports graceful Routing Engine switchover on all routing platforms that contain dual Routing Engines. To support graceful Routing Engine switchover, the DHCP relay agent automatically mirrors (replicates) information about the state of bound DHCP clients from the master Routing Engine to the backup Routing Engine.

To enable graceful Routing Engine switchover support for the extended DHCP relay agent, include the graceful-switchover statement at the [edit chassis redundancy] hierarchy level. You cannot disable graceful Routing Engine switchover support for the extended DHCP relay agent when the router is configured to support graceful Routing Engine switchover.

For more information about using graceful Routing Engine switchover, see the JUNOS High Availability Configuration Guide.