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DHCP Relay Agent

 

The DHCP relay agent operates as the interface between DHCP clients and the server. The DHCP Relay Agent relays DHCP messages between DHCP clients and DHCP servers on different IP networks. For more information, read this topic.

Understanding DHCP Relay Agent Operation

A Juniper Networks device operating as a DHCP relay agent forwards incoming requests from BOOTP and DHCP clients to a specified BOOTP or DHCP server. Client requests can pass through virtual private network (VPN) tunnels.

You cannot configure a single device interface to operate as both a DHCP client and a DHCP relay.

Note

The DHCP requests received on an interface are associated to a DHCP pool that is in the same subnet as the primary IP address/subnet on an interface. If an interface is associated with multiple IP addresses/subnets, the device uses the lowest numerically assigned IP address as the primary IP address/subnet for the interface. To change the IP address/subnet that is listed as the primary address on an interface, use the set interfaces < interface name > unit 0 family inet xxx.xxx.xxx.xxx/yy primary command and commit the change.

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

The pattern of interaction among the DCHP Relay agent, DHCP client, and DHCP servers is the same regardless of whether the software installation is on a router or a switch. However, there are some difference in the details of usage.

On routers—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.

On switches—In a typical network configuration, the DHCP client is on an access device such as a personal computer and the DHCP relay agent is configured on the switch 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.

  1. The DHCP client sends a discover packet to find a DHCP server in the network from which to obtain configuration parameters for the subscriber (or DHCP client), including an IP address.

  2. 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.

  3. 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.

  4. 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.

  5. The DHCP client sends a request packet that specifies the DHCP server from which to obtain configuration information.

  6. The DHCP relay agent receives the request packet and forwards copies to each of the two DHCP servers.

  7. The DHCP server requested by the client sends an acknowledgement (ACK) packet that contains the client’s configuration parameters.

  8. The DHCP relay agent receives the ACK packet and forwards it to the client.

  9. The DHCP client receives the ACK packet and stores the configuration information.

  10. If configured to do so, the DHCP relay agent installs a host route and Address Resolution Protocol (ARP) entry for this client.

  11. 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 (or switch) to determine when the lease for this client has expired or been released. This process is referred to as lease shadowing or passive snooping.

On all Junos OS devices, when the DHCP relay is configured with forward-only option, and the DHCP client is terminated on logical tunnel interface if the logical tunnel interface

  • Includes multiple logical interfaces

  • Use same VLAN on multiple logical interfaces of the same lt interface

In such cases, the DHCP relay might fail to send the OFFER messages.

This issue applies in Junos OS Releases 19.3R3, 19.4R2, 18.4R3, 19.4R1, 19.3R2, 18.4R3-S1, 17.4R3 releases.

Minimum DHCP Relay Agent Configuration

This example shows the minimum configuration you need to use the extended DHCP relay agent on the router or switch:

Note

The interface type in this topic is just an example. The fe- interface type is not supported by EX Series switches.

This example creates a server group and an active server group named test with IP address 203.0.113.21. The DHCP relay agent configuration is applied to a group named all. Within this group, the DHCP relay agent is enabled on interface fe-0/0/2.0.

Configuring DHCP Relay Agent

The DHCP relay agent operates as the interface between DHCP clients and the server. The DHCP Relay Agent relays DHCP messages between DHCP clients and DHCP servers on different IP networks.

This example describes how to configure the DHCP relay agent on the SRX Series device. SRX series device acting as DHCP relay agent is responsible for forwarding the requests and responses between the DHCP clients and the server which are part of different routing instances.

Requirements

This example uses the following hardware and software components:

  • SRX Series devices with Junos OS 15.1X49-D10 or later.

Overview

You can configure DHCP relay agent to provide additional security when exchanging DHCP messages between a DHCP server and DHCP clients that reside in different virtual routing instances. This type of configuration is for DHCP relay connection between a DHCP server and a DHCP client, when the DCHP server resides in a network that is isolated from the client network.

Topology

To exchange DHCP messages between different routing instances, you must enable both the server-facing interface and the client-facing interface of the DHCP relay agent to recognize and forward DHCP packets.

The following Figure 1 shows DHCP performance as DHCP local server, DHCP client, and DHCP relay agent

Figure 1: Understanding DHCP Services in a Routing Instance
Understanding DHCP Services
in a Routing Instance

The following list provides an overview of the tasks required to create the DHCP message exchange between the different routing instances:

  • Configure the client-facing side of the DHCP relay agent.

  • Configure the server-facing side of the DHCP relay agent.

  • Configure the Security Zone to Allow the DHCP protocol.

    Table1: DHCP Relay Parameters:

    Parameters

    Client-Side-Details

    Server-Side-Details

    interface

    ge-0/0/3.0

    ge-0/0/3.0

    routing interface

    trust-vr

    untrust-vr

    ip address

    10.1.1.2/24

    20.1.1.1/24

    Note

    In order to make this setup work, the DHCP server connecting route and relay agent interface route must be in both routing-instances. For example, in the above topology, the server route 30.1.1.0/24 needs to be shared with the dhcp-relay VR, and the dhcp-relay interface route 10.1.1.0/24 exact needs to be shared with the default routing instance.

    Also, a dummy dhcp-relay config must be added in the routing instance with the DHCP server. If this is not configured, dhcp-relay will not be able to receive packets from the DHCP server.

Configuration

CLI Quick Configuration

The following procedures describe the configuration tasks for creating the DHCP message exchange between the DHCP server and clients in different routing instances. To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Quick configuration for Client-Facing Support:

Quick configuration for Server-Facing Support:

Quick configuration for DHCP Relay Support:

Quick configuration for Security Zone to Allow the DHCP Protocol:

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For instructions on how to do that, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure support on the client-facing side of the DHCP relay agent:

  1. Set a routing instance type as virtual router.
  2. Set an interface to the virtual router
  3. Set the IP address to the interface.

Step-by-Step Procedure

To configure support on the server-facing side of the DHCP relay agent:

  1. Set a virtual router.
  2. Set an interface to the virtual router.
  3. Set the forward-only-replies option.
  4. Set the IP address to the interface.

Step-by-Step Procedure

To configure the DHCP local server to support:

  1. Set the configuration in dhcp-relay for untrust-vr routing instance
  2. Set the configuration in dhcp-relay for trust-vr routing instance
  3. Set the configuration to share routes between routing instances.
    Note

    You can enable an SRX Series device to function as a DHCP local server. The DHCP local server provides an IP address and other configuration information in response to a client request.

Step-by-Step Procedure

To configure the security zone to allow the DHCP Protocol:

  1. Set the default security policy to permit all traffic.
  2. Set all system services and protocols on interface ge-0/0/4.0.
  3. Set all system services and protocols on interface ge-0/0/3.0.

Results

  • Result for Client-facing Support:

From configuration mode, confirm your configuration by entering the show routing-instances command. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

  • Result for Server-Facing Support:

    From configuration mode, confirm your configuration by entering the show routing-instances command. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

  • Result for DHCP Local Server Support:

    From configuration mode, confirm your configuration by entering the show routing-instances, show policy-options and show routing-options commands. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

  • Result for Security Zone to Allow the DHCP Protocol:

    From configuration mode, confirm your configuration by entering the show security policies and show security zones commands. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying the DHCP Relay Statistics Configuration:

Purpose

Verify that the DHCP Relay Statistics has been configured.

Action

  • From operational mode, enter the show dhcp relay statistics routing-instance dhcp-relay command.

Verifying DHCP client bindings in the routing instance.

Purpose

Verify that the DHCP client bindings in the routing instances has been configured.

Action

  • From operational mode, enter the show dhcp relay binding routing-instance dhcp-relay command.

Configuring a DHCP Relay Agent on EX Series Switches

You can configure an EX Series switch to act as an extended DHCP relay agent. This means that a locally attached host can issue a DHCP request as a broadcast message and the switch configured for DHCP relay relays the message to a specified DHCP server. Configure a switch to be a DHCP relay agent if you have locally attached hosts and a remote DHCP server.

Before you begin:

  • Ensure that the switch can connect to the DHCP server.

To configure a switch to act as an extended DHCP relay agent server:

  1. Create at least one DHCP server group, which is a group of 1 through 5 DHCP server IP addresses:
  2. Set the global active DHCP server group. The DHCP relay agent relays DHCP client requests to the DHCP servers defined in the active server group:
  3. Create a DHCP relay group that includes at least one interface. DHCP relay runs on the interfaces defined in DHCP groups:
  4. (Optional) Configure overrides of default DHCP relay behaviors, at the global level. See the override options in the overrides statement.
  5. (Optional) Configure DHCP relay to use the DHCP vendor class identifier option (option 60) in DHCP client packets, at the global level:
  6. (Optional) Configure settings for a DHCP relay group that override the settings at the global level, using these statements:
  7. (Optional) Configure settings for a DHCP relay group interface that override the settings at the global and group levels, using these statements:

Configuring DHCP Smart Relay (Legacy DHCP Relay)

You can use DHCP smart relay to provide redundancy and resiliency to your DHCP relay configuration. Smart relay provides additional relay functionality and requires all of the configuration settings required by DHCP relay. To use DHCP smart relay, you also need an interface with multiple IP addresses assigned to it. You can achieve this by doing either of the following tasks:

Once you have created an interface with multiple IP addresses, complete the smart relay configuration by entering one of the following statements:

  • set forwarding-options helpers bootp smart-relay-global: Use this statement to enable smart relay on all the interfaces that are configured as relay agents.

  • set forwarding-options helpers bootp interface interface-name smart-relay-agent: Use this statement to enable smart relay on a specific interface.

When smart relay is configured for an interface, the switch initially sends DHCP request (discover) messages out of that interface using the primary address of the interface as the gateway IP address (in the giaddr field) for the DHCP message. If no DHCP offer message is received from a server in reply, the switch allows the client to send as many as three more discover messages using the same gateway IP address. If no DHCP offer message is received after three retries, the switch resends the discover message using the alternate IP address as the gateway IP address. If you configure more than two IP addresses on the relay agent interface, the switch repeats this process until a DHCP offer message is received or all of the IP addresses have been used without success.

See also

Disabling Automatic Binding of Stray DHCP Requests

DHCP requests that are received but have no entry in the database are known as stray requests. By default, DHCP relay, DHCP relay proxy, and DHCPv6 relay agent attempt to bind the requesting client by creating a database entry and forwarding the request to the DHCP server. If the server responds with an ACK, the client is bound and the ACK is forwarded to the client. If the server responds with a NAK, the database entry is deleted and the NAK is forwarded to the client. This behavior occurs regardless of whether authentication is configured.

You can override the default configuration at the global level, for a named group of interfaces, or for a specific interface within a named group. Overriding the default causes DHCP relay, DHCP relay proxy, and DHCPv6 relay agent to drop all stray requests instead of attempting to bind the clients.

Note

Automatic binding of stray requests is enabled by default.

  • To disable automatic binding behavior, include the no-bind-on-request statement when you configure DHCP overrides at the global, group, or interface level.

  • To override the default behavior for DHCPv6 relay agent, configure the override at the [edit forwarding-options dhcp-relay dhcpv6] hierarchy level.

The following two examples show a configuration that disables automatic binding of stray requests for a group of interfaces and a configuration that disables automatic binding on a specific interface.

To disable automatic binding of stray requests on a group of interfaces:

  1. Specify the named group.
  2. Specify that you want to configure overrides.
  3. Disable automatic binding for the group.

To disable automatic binding of stray requests on a specific interface:

  1. Specify the named group of which the interface is a member.
  2. Specify the interface on which you want to disable automatic binding.
  3. Specify that you want to configure overrides.
  4. Disable automatic binding on the interface.

Using Layer 2 Unicast Transmission instead of Broadcast for DHCP Packets

You can configure the DHCP relay agent to override the setting of the broadcast bit in DHCP request packets. DHCP relay agent then instead uses the Layer 2 unicast transmission method to send DHCP Offer reply packets and DHCP ACK reply packets from the DHCP server to DHCP clients during the discovery process.

To override the default setting of the broadcast bit in DHCP request packets:

  1. Specify that you want to configure override options.
  2. Specify that the DHCP relay agent uses the Layer 2 unicast transmission method.

Changing the Gateway IP Address (giaddr) Field to the giaddr of the DHCP Relay Agent

You can configure the DHCP relay agent to change the gateway IP address (giaddr) field in packets that it forwards between a DHCP client and a DHCP server.

To overwrite the giaddr of every DHCP packet with the giaddr of the DHCP relay agent before forwarding the packet to the DHCP server:

  1. Specify that you want to configure override options.
  2. Specify that the giaddr of DHCP packets is overwritten.

Configure DHCP Relay Agent to Replace Request and Release Packets with Gateway IP address

You can configure the DHCP relay agent to replace request and release packets with the gateway IP address (giaddr) before forwarding the packet to the DHCP server.

To replace the source address with giaddr:

  1. Specify that you want to configure override options.
  2. Specify that you want to replace the IP source address in DHCP relay request and release packets with the gateway IP address (giaddr).

Overriding the Default DHCP Relay Configuration Settings

You can override the default DHCP relay configuration settings at the global level, for a named group of interfaces, or for a specific interface within a named group.

  • To override global default DHCP relay agent configuration options, include the overrides statement and its subordinate statements at the [edit forwarding-options dhcp-relay] hierarchy level.

  • To override DHCP relay configuration options for a named group of interfaces, include the statements at the [edit forwarding-options dhcp-relay group group-name] hierarchy level.

  • To override DHCP relay configuration options for a specific interface within a named group of interfaces, include the statements at the [edit forwarding-options dhcp-relay group group-name interface interface-name] hierarchy level.

  • To configure overrides for DHCPv6 relay at the global level, group level, or per-interface, use the corresponding statements at the [edit forwarding-options dhcp-relay dhcpv6] hierarchy level.

To override default DHCP relay agent configuration settings:

  1. (DHCPv4 and DHCPv6) Specify that you want to configure override options.
    • DHCPv4 overrides.

      Global override:

      Group-level override:

      Per-interface override:

    • DHCPv6 overrides.

      Global override:

      Group-level override:

      Per-interface override:

  2. (DHCPv4 only) Enable DHCP relay proxy mode.

    See Enabling DHCP Relay Proxy Mode.

  3. (DHCPv4 only) Overwrite the giaddr in DHCP packets that the DHCP relay agent forwards.

    See Changing the Gateway IP Address (giaddr) Field to the giaddr of the DHCP Relay Agent.

  4. (DHCPv4 only) Replace the IP source address in DHCP relay request and release packets with the gateway IP address (giaddr).

    See Configure DHCP Relay Agent to Replace Request and Release Packets with Gateway IP address.

  5. (DHCPv4 only) Override the DHCP relay agent information option (option 82) in DHCP packets.

    See Overriding Option 82 Information.

  6. (DHCPv4 only) Override the setting of the broadcast bit in DHCP request packets and use the Layer 2 unicast transmission method.

    See Using Layer 2 Unicast Transmission instead of Broadcast for DHCP Packets.

  7. (DHCPv4 only) Trust DHCP client packets that have a giaddr of 0 and that contain option 82 information.

    See Enable Processing of Untrusted Packets So Option 82 Information Can Be Used.

  8. (DHCPv4 and DHCPv6) Override the maximum number of DHCP clients allowed per interface.

    See Specifying the Maximum Number of DHCP Clients Per Interface.

  9. (DHCPv4 only) Configure client auto logout.

    See DHCP Auto Logout Overview.

  10. (DHCPv4 and DHCPv6) Enable or disable support for DHCP snooped clients on interfaces.

    See Enabling and Disabling DHCP Snooped Packets Support for DHCP Relay Agent.

  11. (DHCPv4 and DHCPv6) Delay authentication of subscribers until the DHCP client sends a Request packet.

    See the delay-authentication.

  12. (DHCPv4 and DHCPv6) Send release messages to the DHCP server when clients are deleted.

    See Sending Release Messages When Clients Are Deleted.

  13. (Optional) Specify that when the DHCP or DHCPv6 relay agent receives a Discover or Solicit message that has a client ID that matches the existing client entry, the relay agent deletes the existing client entry.

    See DHCP Behavior When Renegotiating While in Bound State.

  14. (DHCPv6 only) Automatically log out existing client when new client solicits on same interface.

    See Automatically Logging Out DHCPv6 Clients.

  15. (DHCPv4 only) Disable the DHCP relay agent on specific interfaces.

    See Disabling DHCP Relay Agent for Interfaces, for Groups, or Globally.

  16. (DHCPv4 and DHCPv6) Disable automatic binding of stray DHCP requests.

    See Disabling Automatic Binding of Stray DHCP Requests.

  17. (DHCPv4 and DHCPv6) Assign a single-session DHCP dual-stack group to a specified group of subscribers. You must assign the group to both legs of the DHCP dual stack.

    See Configuring Single-Session DHCP Dual-Stack Support.

  18. (Optional, DHCPv4 and DHCPv6l) Specify that a short lease be sent to the client.

    See Configuring DHCP Asymmetric Leasing.

Disabling DHCP Relay Agent for Interfaces, for Groups, or Globally

You can disable DHCP relay on all interfaces or a group of interfaces.

To disable DHCP relay agent:

  1. Specify that you want to configure override options.
  2. Disable the DHCP relay agent.

Example: Configuring DHCP Relay Agent Selective Traffic Processing Based on DHCP Option Strings

This example shows how to configure DHCP relay agent to use DHCP option strings to selectively identify, filter, and process client traffic.

Requirements

This example uses the following hardware and software components:

  • MX Series 5G Universal Routing Platforms or EX Series Switches

Before you configure DHCP relay agent selective processing support, be sure you:

Overview

In this example, you configure DHCP relay agent to use DHCP option strings in client packets to selectively identify, filter, and process client traffic. To configure selective processing, you perform the following procedures:

  1. Identify the client traffic—Specify the DHCP option that DHCP relay agent uses to identify the client traffic you want to process. The option you specify matches the option in the client traffic.

  2. Configure a default action—Specify the default processing action, which DHCP relay uses for identified client traffic that does not satisfy any configured match criteria.

  3. Create match filters and associate an action with each filter—Specify match criteria that filter the client traffic. The criteria can be an exact match or a partial match with the option string in the client traffic. Associate a processing action with each match criterion.

Configuration

To configure DHCP relay agent selective processing based on DHCP option information, perform these tasks:

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them in a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the command into the CLI at the [edit] hierarchy level.

Configuring DHCP Relay Agent To Selectively Process Client Traffic Based on DHCP Option Strings

Step-by-Step Procedure

To configure DHCP relay selective processing:

  1. Specify that you want to configure DHCP relay agent support.
  2. Specify the DHCP option that DHCP relay agent uses to identify incoming client traffic.
  3. Configure a default action, which DHCP relay agent uses when the incoming client traffic does not satisfy any configured match criteria.
  4. Configure an exact match condition and associated action that DHCP relay uses to process the identified client traffic.
  5. Configure a second exact match condition and associated action that DHCP relay uses to process client traffic.
  6. Configure a partial match criteria and associated action that DHCP relay uses to process client traffic.

Results

From configuration mode, confirm the results of your configuration by issuing the show statement at the [edit forwarding-options] hierarchy level. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

If you are done configuring the device, enter commit from configuration mode.

Verification

To verify the status of DHCP relay agent selective traffic processing, perform this task:

Verifying the Status of DHCP Relay Agent Selective Traffic Processing

Purpose

Verify the DHCP relay agent selective traffic processing status.

Action

Display statistics for DHCP relay agent.

user@host> show dhcp relay statistics

Meaning

The Packets forwarded field in the show dhcp relay statistics command output displays the number of client packets that have been forwarded as a result of the selective traffic processing configuration. In this example, the output indicates the total number of packets that DHCP relay agent has forwarded, as well as a breakdown for the number of BOOTREQUEST and BOOTREPLY packets forwarded.

Verifying and Managing DHCP Relay Configuration

Purpose

View or clear address bindings or statistics for DHCP relay agent clients.

Action

  • To display the address bindings for DHCP relay agent clients:

  • To display DHCP relay agent statistics:

  • To clear the binding state of DHCP relay agent clients:

  • To clear all DHCP relay agent statistics:

To clear or view information about client bindings and statistics in a routing instance, run the following commands:

  • show dhcp relay binding routing instance <routing-instance name>

  • show dhcp relay statistics routing instance <routing-instance name>

  • clear dhcp relay binding routing instance <routing-instance name>

  • clear dhcp relay statistics routing instance <routing-instance name>

Note

On all SRX Series devices, DHCP relay is unable to update the binding status based on DHCP_RENEW and DHCP_RELEASE messages.

Extended DHCP Relay Agent Overview

You can configure extended DHCP relay options on the router or on the switch and enable the router (or switch) 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.

DHCP relay supports the attachment of dynamic profiles and also interacts with the local AAA Service Framework to use back-end authentication servers, such as RADIUS, to provide subscriber authentication or DHCP client authentication. You can attach dynamic profiles and configure authentication support on a global basis or for a specific group of interfaces.

Note

The PTX Series Packet Transport Routers do not support authentication for DHCP relay agents.

On the routers, you can use DHCP relay in carrier edge applications such as video/IPTV to obtain configuration parameters, including an IP address, for your subscribers.

On the switches, you can use DHCP relay to obtain configuration parameters including an IP address for DHCP clients.

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

You can also configure the extended DHCP relay agent to support IPv6 clients. SeeDHCPv6 Relay Agent Overview for information about the DHCPv6 relay agent feature.

To configure the extended DHCP relay agent on the router (or switch), include the dhcp-relay statement at the [edit forwarding-options] hierarchy level.

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]

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

The pattern of interaction among the DCHP Relay agent, DHCP client, and DHCP servers is the same regardless of whether the software installation is on a router or a switch. However, there are some difference in the details of usage.

On routers—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.

On switches—In a typical network configuration, the DHCP client is on an access device such as a personal computer and the DHCP relay agent is configured on the switch 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.

  1. The DHCP client sends a discover packet to find a DHCP server in the network from which to obtain configuration parameters for the subscriber (or DHCP client), including an IP address.

  2. 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.

  3. 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.

  4. 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.

  5. The DHCP client sends a request packet that specifies the DHCP server from which to obtain configuration information.

  6. The DHCP relay agent receives the request packet and forwards copies to each of the two DHCP servers.

  7. The DHCP server requested by the client sends an acknowledgement (ACK) packet that contains the client’s configuration parameters.

  8. The DHCP relay agent receives the ACK packet and forwards it to the client.

  9. The DHCP client receives the ACK packet and stores the configuration information.

  10. If configured to do so, the DHCP relay agent installs a host route and Address Resolution Protocol (ARP) entry for this client.

  11. 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 (or switch) to determine when the lease for this client has expired or been released. This process is referred to as lease shadowing or passive snooping.

DHCP Liveness Detection

Liveness detection for DHCP subscriber or DHCP client IP sessions utilizes an active liveness detection protocol to institute liveness detection checks for relevant clients. Clients are expected to respond to liveness detection requests within a specified amount of time. If the responses are not received within that time for a given number of consecutive attempts, then the liveness detection check fails and a failure action is implemented.

Note

DHCP liveness detection either globally or per DHCP group.