DHCP for Switches

Understanding DHCP Services for Switches

• DHCP Client/Server Model

• Using DHCP

• DHCP Relay Servers and DHCP Servers

• Legacy DHCP and Extended DHCP for Server Versions

• Configuring DHCP on a Switch

• How DHCP Works

DHCP Client/Server Model

DHCP IP address allocation works on a client/server model in which the server, in this case a switch, assigns the client reusable IP information from an address pool. A DHCP client might receive offer messages from multiple DHCP servers and can accept any one of the offers; however, the client usually accepts the first offer it receives. See Figure 1.

Figure 1: DHCP Client/Server Model

Using DHCP

DHCP automates network-parameter assignment to network devices. Even in small networks, DHCP is useful because it makes it easy to add new machines to the network.

DHCP access service minimizes the overhead required to add clients to the network by providing a centralized, server-based setup, which means that you do not have to manually create and maintain IP address assignments for clients. In addition, when you use DHCP to manage a pool of IP addresses among hosts, you reduce the number of IP addresses needed on the network. DHCP does this by leasing an IP address to a host for a limited period of time, allowing the DHCP server to share a limited number of IP addresses. DHCP also provides a central database of devices that are connected to the network and eliminates duplicate resource assignments. In addition to IP addresses for clients, DHCP provides other configuration information, particularly the IP addresses of local caching Domain Name System (DNS) resolvers, network boot servers, or other service hosts.

Another valuable DHCP feature is automatic software download for installation of software packages on switches. DHCP clients configured for automatic software download receive messages as part of the DHCP message exchange process—when the software package name in the DHCP server message is different from that of the software package that booted the DHCP client switch, the new software is downloaded and installed. See Upgrading Software by Using Automatic Software Download for Switches.

DHCP Relay Servers and DHCP Servers

You can configure a switch either as a DHCP server or as a DHCP relay server, but not both. Whereas a DHCP server replies to a client with an IP address, a DHCP relay server relays DHCP messages to and from the configured DHCP server, even if the client and server are on different IP networks.

Configure a switch to be a DHCP relay agent if you have locally attached hosts and a remote DHCP server.

Legacy DHCP and Extended DHCP for Server Versions

Two versions of both DHCP server and DHCP relay agent are available on EX Series, QFX Series, and OCX Series switches. The original legacy DHCP server and legacy DHCP relay agent can be used in the same network as the extended DHCP servers and extended DHCP relay agent—extended DHCP is also referred to as virtual router (VR) aware DHCP.

You cannot configure legacy DHCP and extended DHCP versions on the same switch. Because the newer extended DHCP server version has more features, we recommend that you configure the extended DHCP server if it is supported by the switch.

The extended DHCP server version has the following added features:

• Graceful Routing Engine switchover (GRES), which provides mirroring support for clients.

• Virtual routing and forwarding (VRF), which allows multiple instances of a routing table to simultaneously coexist on the same switch. For details, see Understanding Virtual Routing Instances on EX Series Switches .

Legacy DHCP and extended DHCP servers can be configured at the hierarchy levels shown in Table 1:

DHCP clients on a switch are always configured at the hierarchy level [edit interfaces interface-name family dhcp].

Configuring DHCP on a Switch

A DHCP configuration consists of two parts: the configuration for a DHCP server and the configuration for DHCP clients. The DHCP server configuration is simple if you accept the default configurations.

When you configure a legacy DHCP server, you only need to define the DHCP server name and the interface on the switch. You can use the default configuration for the rest of the settings. When you configure an extended DHCP server, you need to only define a DHCP pool, indicate IP addresses for the pool, and create a server group. You can use the default configuration for the rest of the settings.

For directions for configuring either a legacy DHCP server or an extended DHCP server, see Configuring a DHCP Server on Switches (CLI Procedure).

To configure a DHCP client, set the client’s DHCP interface address in the [edit interfaces interface-name unit 0 family inet dhcp] hierarchy. For directions for configuring a DHCP client on a switch, see Configuring a DHCP Client (CLI Procedure).

How DHCP Works

DHCP consists of a four-step transfer process beginning with a broadcast DHCP discovery message from the client. As the second step, the client receives a DHCP offer message from the server. This message includes the IP address and mask, and some other specific parameters. The client then sends a DHCP request message to accept the IP address and other parameters that it received from the server in the previous step. The DHCP server sends a DHCP response message and removes the now-allocated address from the DHCP address pool. See Figure 2.

Figure 2: DHCP Four-Step Transfer

Configuring a Switch as a DHCP Server (CLI Procedure)

This topic describes the following task:

1. Configuring the Switch as a Local DHCP Server

Configuring the Switch as a Local DHCP Server

1. Configure a Layer 3 interface with an IP address on which the DHCP server will be reachable:
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   user@switch# set interfaces interface-name unit unit-number family family address address/prefix-length

   user@switch# set vlans vlan-name vlan-id vlan-id

   user@switch# set vlans vlan-name l3-interface irb-name

   user@switch# set interfaces irb-name family family address address/prefix-length

   For example:

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   user@switch# set interfaces ge-0/0/1 unit 0 family inet address 192.0.2.1/24

   user@switch# set vlans server vlan-id 301

   user@switch# set vlans server l3-interface irb.301

   user@switch# set interfaces irb.301 family inet address 192.0.2.2/24
2. Configure the DHCP server for the Layer 3 interface:
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   user@switch# set system services dhcp-local-server group-name interface interface-name

   For example:

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   user@switch# set system services dhcp-local-server group server1 interface ge-0/0/1

   user@switch# set system services dhcp-local-server group server1 interface irb.301
3. Create an address pool for IPv4 addresses that can be assigned to clients. The addresses in the pool must be on the subnet in which the DHCP clients reside. Do not include addresses that are already in use on the network.
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   user@switch# set access address-assignment pool pool-name family family network address/prefix-length

   For example:

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   user@switch# set access address-assignment pool pool1 family inet network 198.51.100.0/24
4. (Optional) Define a range of addresses in the address-assignment pool. The range is a subset of addresses within the pool that can be assigned to clients. If no range is specified, then all addresses within the pool are available for assignment. Configure the name of the range and the lower and upper boundaries of the addresses in the range:
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   user@switch# set access address-assignment pool pool-name family family range range-name low low-IP-address

   user@switch# set access address-assignment pool pool-name family family range range-name high high-IP-address

   For example:

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   user@switch# set access address-assignment pool pool1 family inet range range1 low 198.51.100.1

   user@switch# set access address-assignment pool pool1 family inet range range1 high 198.51.100.2
5. (Optional) Configure one or more routers as the default gateway on the client’s subnet:
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   user@switch# set access address-assignment pool pool-name family family dhcp-attributes router gateway-ip-address

   For example:

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   user@switch# set access address-assignment pool pool1 family inet dhcp-attributes router 198.1.1.254
6. (Optional) Configure the IP address that is used as the source address for the DHCP server in messages exchanged with the client. Clients use this information to distinguish between lease offers.
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   user@switch# set access address-assignment pool pool-name family family dhcp-attributes server-identifier ip-address

   For example:

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   user@switch# set access address-assignment pool pool1 family inet dhcp-attributes server-identifier 198.51.100.254
7. (Optional) Specify the maximum time period, in seconds, that a client holds the lease for an assigned IP address if the client does not renew the lease:
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   user@switch# set access address-assignment pool pool-name family family dhcp-attributes maximum-lease-time seconds

   For example:

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   user@switch# set access address-assignment pool pool1 family inet dhcp-attributes maximum-lease-time 43,200
8. (Optional) Specify user-defined options to be included in DHCP packets:
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   user@switch# set access address-assignment pool pool-name family family dhcp-attributes option option-id-number option-type option-value

   For example:

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   user@switch# set access address-assignment pool pool1 family inet dhcp-attributes option 98 string test98

Configuring a DHCP Server on Switches (CLI Procedure)

This topic includes the following tasks:

1. Configuring an Extended DHCP Server on a Switch

2. Configuring a Legacy DHCP Server on a Switch (CLI Procedure)

Configuring an Extended DHCP Server on a Switch

1. Create an address pool for DHCP IP addresses:
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   user@switch# set access address-pool address-pool
2. Configure an address-assignment pool that can be used by different client applications for DHCP dynamic assignment:
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   [edit access address-assignment]

   user@switch# set pool address-pool-name
3. Create a server group on the switch, providing a group name and an interface name for DHCP:
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   [edit system services dhcp-local-server]

   user@switch# set group group-name interface interface-name
4. (Optional) Process the information protocol data units (PDUs):
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   [edit system services dhcp-local-server]

   user@switch# set overrides process-inform
5. (Optional) Redefine the order of attribute matching for pool selection:
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   [edit system services dhcp-local-server]

   user@switch# set pool-match-order ip-address-first
6. (Optional) Enable dynamic reconfiguration triggered by the DHCP extended server for all DHCP clients or only for the DHCP clients serviced by the specified group of interfaces:
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   [edit system services dhcp-local-server]

   user@switch# set reconfigure
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   [edit system services dhcp-local-server group group-name]

   user@switch# set reconfigure

Configuring a Legacy DHCP Server on a Switch (CLI Procedure)

To configure a legacy DHCP server, you must configure a pool of IP addresses for dynamic assignment. You only need to supply a series of network addresses. Additional configurations are optional.

1. Configure a pool of IP addresses for dynamic assignment:
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   [edit system services dhcp]

   user@switch# set pool network-range
   Note

   Step 2 through Step 15 are for assigning global values at the[edit system services dhcp] hierarchy level. You can also assign the same values to a specific pool by using those same commands at the [edit system services dhcp pool network-range] hierarchy level.

2. (Optional) Change the domain search list used to resolve hostnames:
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   [edit system services dhcp]

   user@switch# set domain-search [ domain-list ]
3. (Optional) Change the domain name server (DNS) name that the DHCP server advertises to clients:
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   [edit system services dhcp]

   user@switch# set name-server address
4. (Optional) Change the DHCP options:
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   [edit system services dhcp]

   user@switch# set option id-number
5. (Optional) Change the devices advertised to clients:
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   [edit system services dhcp]

   user@switch# set router address
6. (Optional) Configure the name of the boot server advertised to DHCP clients. The client uses a boot file located on the boot server to complete the DHCP setup. This configuration step is equivalent to DHCP Option 66:
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   [edit system services dhcp]

   user@switch# set boot-server (address | hostname)
7. (Optional) Set the boot file advertised to DHCP clients. After the client receives an IP address and the boot file location from the DHCP server, the client uses the boot image stored in the boot file to complete DHCP setup. This configuration step is equivalent to DHCP Option 67:
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   [edit system services dhcp]

   user@switch# set boot-file filename
8. (Optional) Change the SIP server:
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   [edit system services dhcp]

   user@switch# set sip-server addresses-or-names

   For more information, see Configuring a DHCP SIP Server (CLI Procedure).

9. (Optional) Change the DHCP client’s hardware address:
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   [edit system services dhcp]

   user@switch# set static-binding mac-address
10. (Optional) Change the NetBIOS name server:
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    [edit system services dhcp]

    user@switch# set wins-server address

Configuring a DHCP Client (CLI Procedure)

DHCP configuration consists of two components, configuration of DHCP clients and configuration of a DHCP server. Client configuration determines how clients send a message requesting an IP address, whereas a DHCP server configuration enables the server to send an IP address configuration back to the client. This topic describes configuring a DHCP client. For directions for configuring a DHCP server, see Configuring a DHCP Server on Switches (CLI Procedure) or Configuring a Switch as a DHCP Server (CLI Procedure).

You can change DHCP client configurations from the switch, using client identifiers to indicate which clients you want to configure.

To configure a DHCP client, you configure an interface to belong to the DHCP family and specify additional attributes, as desired:

[edit]
user@switch# set interfaces interface-name unit number family  inet dhcp
 configuration-statement

The options that you can configure are listed in Table 2. Replace the variable configuration-statement with one or more of the statements listed in this table. If you do not explicitly configure these options, the switch uses default values for them.

Configuring a DHCP SIP Server (CLI Procedure)

You can use the sip-server statement on the EX Series switch to configure option 120 on a DHCP server. The DHCP server sends configured option values—Session Initiation Protocol (SIP) server addresses or names—to DHCP clients when they request them. Previously, you were only allowed to specify a SIP server by address using [edit system services dhcp option 120]. You specify either an IPv4 address or a fully qualified domain name to be used by SIP clients to locate a SIP server. You cannot specify both an address and name in the same statement.

To configure a SIP server using the address option:

For example, to configure one address:

To configure a SIP server using the name option:

For example, to configure a name:

DHCP and BOOTP Relay Overview

You can also use smart DHCP relay, which enables you to configure alternative IP addresses for the gateway interface so that if the server fails to reply to the requests sent from the primary gateway address, the switch can resend the requests using the alternative gateway addresses. To use this feature, you must configure a Layer 3 interface, Layer 3 subinterface, or IRB interface with multiple IP addresses and configure that interface to be a relay agent.

Configuring DHCP and BOOTP

You can configure a switch to act as a Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) server or DHCP relay agent. When a switch is a relay agent, if a locally attached host issues a DHCP or BOOTP request as a broadcast message, the switch relays the message to a specified DHCP or BOOTP server. You should configure a switch to be a DHCP and BOOTP relay agent if you have locally attached hosts and a remote DHCP or BOOTP server.

To configure a switch to be a server, use the dhcp-local-server statement. To configure a switch to be a relay agent, use the dhcp-relay statement.

If you want to enable BOOTP support when the switch is configured to be a DHCP server, enter the following statement:

If you want to enable BOOTP support when the switch is configured to be a DHCP relay agent, enter the following statement:

Graceful Routing Engine Switchover for DHCP

For EX Series switches, only extended DHCP local server maintains the state of active DHCP client leases. The DHCP local server 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. You can configure dynamic profile and authentication support on a global basis or for a specific group of interfaces. The extended DHCP local server also supports the use of Junos address-assignment pools or external authorities, such as RADIUS, to provide the client address and configuration information.

For MX Series routers, the extended DHCP local server and the DHCP relay agent applications both maintain the state of active DHCP client leases in the session database. The extended DHCP application can recover this state if the DHCP process fails or is manually restarted, thus preventing the loss of active DHCP clients in either of these circumstances. However, the state of active DHCP client leases is lost if a power failure occurs or if the kernel stops operating (for example, when the router is reloaded) on a single Routing Engine.

You can enable graceful switchover support on both EX Series switches and MX Series routers. To enable graceful switchover support for the extended DHCP local server or extended DHCP relay agent on a switch, include the graceful-switchover statement at the [edit chassis redundancy] hierarchy level. To enable graceful Routing Engine switchover support on MX Series routers, include the graceful-switchover statement at the [edit chassis redundancy] hierarchy level. You cannot disable graceful Routing Engine switchover support for the extended DHCP application when the router is configured to support graceful Routing Engine switchover.

For more information about using graceful Routing Engine switchover, see Understanding Graceful Routing Engine Switchover.

Centrally Configured Opaque DHCP Options

• Data Flow for RADIUS-Sourced DHCP Options

• Multiple VSA 26-55 Instances Configuration

• DHCP Options That Cannot Be Centrally Configured

Data Flow for RADIUS-Sourced DHCP Options

Figure 3 shows the procedure subscriber management (DHCP management) uses when configuring DHCP options for subscribers (DHCP clients).

Figure 3: DHCP Options Data Flow

1. The subscriber (DHCP client) sends a DHCP discover message (or DHCPv6 solicit message) to the DHCP local server. The message includes client-sourced DHCP options.
2. The DHCP local server initiates authentication with the Junos OS RADIUS client.
3. The RADIUS client sends an Access-Request message on behalf of the subscriber (DHCP client) to the external RADIUS server. The message includes the subscriber’s (DHCP client’s) client-sourced DHCP options.
4. The external RADIUS server responds by sending an Access-Accept message to the RADIUS client. The Access-Accept message includes the RADIUS-sourced opaque DHCP options in VSA 26-55.
5. The RADIUS client sends the DHCP options string to DHCP local server. If there are multiple VSA 26-55 instances, the RADIUS client first assembles them into a single options string.
6. DHCP local server processes all options into the DHCP offer (or DHCPv6 reply) message, except for the RADIUS-sourced VSA 26-55 DHCP options. After processing all other options, DHCP local server then appends the unmodified VSA 26-55 DHCP options to the message and sends the message to the subscriber (DHCP client).
7. The subscriber (DHCP client) is configured with the DHCP options.
8. The following operations occur after the subscriber (DHCP client) receives the DHCP options:

   • Accounting—The RADIUS client sends Acct-Start and Interim-Accounting requests to the RADIUS server, including the RADIUS-sourced DHCP options in VSA 26-55. By default, the DHCP options are included in accounting requests.

   • Renewal—When the subscriber (DHCP client) renews, the cached DHCP options value is returned in the DHCP renew (or DHCPv6 ACK) message. The originally assigned DHCP options cannot be modified during a renew cycle.

   • Logout—When the subscriber (DHCP client) logs out, the RADIUS client sends an Acct-Stop message to the RADIUS server, including the RADIUS-sourced VSA 26-55.

Multiple VSA 26-55 Instances Configuration

VSA 26-55 supports a maximum size of 247 bytes. If your RADIUS-sourced DHCP options field is greater than 247 bytes, you must break the field up and manually configure multiple instances of VSA 26-55 for the RADIUS server to return. When using multiple instances for an options field, you must place the instances in the packet in the order in which the fragments are to by reassembled by the RADIUS client. The fragments can be of any size of 247 bytes or less.

When the RADIUS client returns a reassembled opaque options field in an accounting request to the RADIUS server, the client uses 247-byte fragments. If you had originally created instances of fewer than 247 bytes, the returned fragments might not be the same as you originally configured on the RADIUS server.

DHCP Options That Cannot Be Centrally Configured

Table 3 shows the DHCP options that you must not centrally configure on the RADIUS server.