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IP Address Assignment Pool

Address pool is a set of Internet Protocol (IP) addresses available for allocation to users, such as in host configurations with the DHCP. An address-assignment pool can support either IPv4 address or IPv6 addresses. You can create centralized IPv4 and IPv6 address pools independently of the client applications that use the pools. For more information, read this topic.

Address-Assignment Pools Overview

The address-assignment pool enables you to create centralized IPv4 and IPv6 address pools independent of the client applications that use the pools. The authd process manages the pools and the address allocation, whether the addresses come from local pools or from a RADIUS server.

For example, multiple client applications, such as DHCP, can use the same address-assignment pool to provide addresses for their particular clients. Client applications can acquire addresses for either authenticated or unauthenticated clients. The pool selected for a subscriber, based on the RADIUS server or network matching or other rule, is called the matching pool for the subscriber.

Address Assignment Types

Address-assignment pools support both dynamic and static address assignment. In dynamic address assignment, a client is automatically assigned an address from the address-assignment pool. In static address assignment, which is supported for IPv4 pools only, you reserve an address that is then always used by a particular client. Addresses that are reserved for static assignment are removed from the dynamic address pool and cannot be assigned to other clients.

Named Address Ranges in Address Assignment Pool

You can configure named address ranges within an address-assignment pool. A named range is a subset of the overall address range. A client application can use named ranges to manage address assignment based on client-specific criteria. For example, for IPv4 address-assignment pools, you might create a named range that is based on a specific DHCP option 82 value. Then, when a DHCP client request matches the specified option 82 value, an address from the specified range is assigned to the client.

Address Allocation from Linked Address Pools

You can link address-assignment pools together to provide backup pools for address assignment. When no addresses are available in the primary or in the matching address pool, the device automatically proceeds to the linked (secondary) address pool to search for an available address to allocate.

Although the first pool in a chain of linked pools is generally considered the primary pool, a matching pool is not necessarily the first pool in the chain.

Starting in Junos OS Release 18.1R1, search mechanism for an available address proceeds through a chain of linked pools. This behavior enables the DHCP to search addresses contiguously..

Lets use an example on how the search mechanism works. Consider a chain of three pools— A, B, and C. Pool A is the primary pool, Pool B is the matching pool for certain subscribers based on information returned by the RADIUS server. The search for an available address for those subscribers uses the following sequence:

  • By default, the matching pool (Pool B) is searched first.

  • The search moves to the first pool (Pool A) in the chain if address not found.

  • The search proceeds through the chain (Pool C) until an available address is found and allocated, or until the search determines no addresses are free.

  • In each pool, all address ranges are fully searched for an address.

You can configure the linked-pool-aggregation statement to start searching within a block of addresses in each range in the matching pool and then successively through the linked pools. The search then moves back to the first pool in the chain and searches all addresses in all ranges in each pool through the last pool in the chain.

Address Pool Hold-Down State

You can configure an address-assignment pool in hold-down state. When the address pool is in hold-down state, the pool is no longer available to allocate IP addresses for the subscribers. This configuration gracefully transforms the active pool to an inactive state as the previously allocated addresses are returned to the pool. When the pool is inactive, you can safely perform maintenance on the pool without affecting any active subscribers.

Address-Assignment Pool for Neighbor Discovery Router Advertisement

You can explicitly allocate an address-assignment pool for Neighbor Discovery Router Advertisement (NDRA).

Excluding Specified Address or Address Range

Starting in Junos OS Release 18.1R1, you can exclude a specified address or range of consecutive addresses to prevent them from being allocated from an address pool.

For example, you might want to reserve certain addresses or ranges to be used only for static subscribers. When you configure an address or range to be excluded, and the address or an address within the range, has already been allocated, that subscriber is logged out, the address is deallocated, and the address is marked for exclusion.

Licensing Requirement

This feature requires a license. To understand more about Subscriber Access Licensing , see Subscriber Access Licensing Overview. Please refer to the Juniper Licensing Guide for general information about License Management. Please refer to the product Data Sheets for details, or contact your Juniper Account Team or Juniper Partner.

Benefits of Address Assignment Pools

  • The address-assignment pool feature supports both subscriber management and DHCP management.

  • You can create centralized pools of addresses independent of client applications.

  • You can specify blocks of addresses, named ranges, so that a given address pool can be used to supply different addresses for different client applications or for subscribers that match different sets of criteria.

  • You can link pools together to ensure that pools are searched for free addresses in a specific manner, contiguously or noncontiguously.

  • You can gracefully transition an address pool from active to inactive by specifying that no further addresses are allocated from the pool.

Extended DHCP Local Server and Address-Assignment Pools

The extended DHCP local server enhances traditional DHCP server operation in which the client address pool and client configuration information reside on the DHCP server. With the extended DHCP local server, the client address and configuration information reside in centralized address-assignment pools, which are managed independently of the DHCP local server and which can be shared by different client applications.

The extended DHCP local server also supports advanced pool matching and the use of named address ranges. You can also configure the local server to use DHCP option 82 information in the client PDU to determine which named address range to use for a particular client. The client configuration information, which is configured in the address-assignment pool, includes user-defined options, such as boot server, grace period, and lease time.

Configuring the DHCP environment that includes the extended DHCP local server requires two independent configuration operations, which you can complete in any order.

In one operation, you configure the extended DHCP local server on the router and specify how the DHCP local server determines which address-assignment pool to use.

In the other operation, you configure the address-assignment pools used by the DHCP local server. The address-assignment pools contain the IP addresses, named address ranges, and configuration information for DHCP clients. See Address-Assignment Pool Configuration Overview for details about creating and using address-assignment pools.

Note:

The extended DHCP local server and the address-assignment pools used by the server must be configured in the same logical system and routing instance.

Interaction Among the DHCP Client, Extended DHCP Local Server, and Address-Assignment Pools

The pattern of interaction between the DHCP local server, the DHCP client, and address-assignment pools is the same regardless of whether you are using a router or a switch. However, there are some differences in the details of usage.

  • On routers—In a typical carrier edge network configuration, the DHCP client is on the subscriber’s computer or customer premises equipment (CPE), and the DHCP local server is configured on the router.

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

The following steps provide a high-level description of the interaction among the DHCP local server, DHCP client, and address-assignment pools:

  1. The DHCP client sends a discover packet to one or more DHCP local servers in the network to obtain configuration parameters and an IP address for the subscriber (or DHCP client).

  2. Each DHCP local server that receives the discover packet then searches its address-assignment pool for the client address and configuration options. Each local server creates an entry in its internal client table to keep track of the client state, then sends a DHCP offer packet to the client.

  3. On receipt of the offer packet, the DHCP client selects the DHCP local server from which to obtain configuration information and sends a request packet indicating the DHCP local server selected to grant the address and configuration information.

  4. The selected DHCP local server sends an acknowledgement packet to the client that contains the client address lease and configuration parameters. The server also installs the host route and ARP entry, and then monitors the lease state.

Configuring How the Extended DHCP Local Server Determines Which Address-Assignment Pool to Use

You can specify the match order in which the extended DHCP local server uses the client data to determine the address-assignment pool that provides the IP address and configuration for a DHCP client. If you do not specify any pool match order, the device uses the default IP address configured in IP address first matching option to select the address pool.

Example:

You can specify the order for pool matching methods. You can specify the methods in any order. All methods are optional. IP address first method is default method.

  • IP address first—Default option. The server selects the address-assignment pool to use by matching the IP address in the client DHCP request with the network address of the address-assignment pool.

    • If the client request contains the gateway IP address (giaddr), the local server matches the giaddr to the address-assignment pool’s address.

    • If the client request does not contain the giaddr, then the DHCP local server matches the IP address of the receiving interface to the address of the address-assignment pool.

    Example:

  • External authority—The DHCP local server receives the address assignment from an external authority, such as RADIUS or Diameter.

    • If RADIUS is the external authority, the DHCP local server uses the Framed-IPv6-Pool attribute (RADIUS attribute 100) to select the pool.

    • If Diameter is the external authority, the server uses the Diameter counterpart of the Framed-IPv6-Pool attribute to determine the pool.

    Example:

  • Option 82—For IPv4 address-Extended DHCP local server matches the DHCP relay agent information option (option 82) in the client DHCP packets to a named range in the address-assignment pool. Named ranges are subsets within the overall address-assignment pool address range, which you can configure when you create the address-assignment pool.

    Example:

    To use the DHCP local server option 82 matching feature with an IPv4 address-assignment pool, you must ensure that the option-82 statement is included in the dhcp-attributes statement for the address-assignment pool.

This example shows an extended DHCP local server configuration that includes optional IPv4 address-assignment pool matching and interface groups. For pool matching, this configuration specifies that the DHCP local server first check the response from an external authentication authority (for example, RADIUS) and use the Framed-IPv6-Pool attribute to determine the address-assignment pool to use for the client address. If no external authority match is found, the DHCP local server then uses ip-address-first matching together with the option 82 information to match the named address range for client IPv4 address assignment. The option 82 matching must also be included in the address-assignment pool configuration.

Assign a Specific IP Address to a Client Using DHCP Option 50 and DHCPv6 IA_NA Option

Subscriber management or DHCP management enables you to specify that DHCP local server assign a particular address to a client. For example, if a client is disconnected, you might use this capability to assign the same address that the client was using prior to being disconnected. If the requested address is available, DHCP assigns it to the client. If the address is unavailable, the DHCP local server offers another address, based on the address allocation process.

Both DHCP local server and DHCPv6 local server support the specific address request feature. DHCP local server uses DHCP option 50 in DHCP discover messages to request a particular address, while DHCPv6 local server uses the IA_NA option (Identity Association for Non-Temporary Addresses) in DHCPv6 solicit messages.

Note:

Subscriber management (DHCP management) supports only one address for each of the DHCPv6 IA_NA or IA_PD address types. If the DHCPv6 client requests more than one address for a given type, the DHCPv6 local server uses only the first address and ignores the other addresses.

Configuring Address-Assignment Pools

The address-assignment pool feature enables you to create address pools that can be shared by different client applications such as DHCPv4 or DHCPv6.

To configure an address-assignment pool, use the following order. The following procedures are tested on for SRX300, SRX320, SRX340, SRX345, SRX550M, and SRX1500 devices.

  1. Configure the address-assignment pool name and specify the addresses for the pool.
  2. (Optional) Configure named ranges (subsets) of addresses.
  3. (Optional; IPv4 only) Create static address bindings.
  4. (Optional) Configure attributes for DHCP clients.

Configuring an Address-Assignment Pool Name and Addresses

When configuring an address-assignment pool on devices, you must specify the name of the pool and its addresses.

To configure an IPv4 address-assignment pool:

  1. Configure the name of the pool and specify the IPv4 family.
  2. Configure the network address and the prefix length of the addresses in the pool.
Note:

You can configure an IPv4 address-assignment pool in a routing instance by configuring the address-assignment statements at the[edit routing-instance routing-instance-name] hierarchy level. For example [edit routing-instances routing-instances name access address-assignment pool blr-pool family inet]. The above steps shows only the [edit access] configuration.

Configuring a Named Address Range for Dynamic Address Assignment

You can optionally configure multiple named ranges, or subsets, of addresses within an address-assignment pool. During a dynamic address assignment, a client can be assigned an address from a specific named range. To create a named range, you specify a name for the range and define the address range.

This example is tested on SRX300, SRX320, SRX340, SRX345, SRX1500, and SRX550M devices.

To create a named range within an IPv4 address-assignment pool:

  1. Specify the name of the address-assignment pool.
  2. Configure the name of the range and the lower and upper boundaries of the addresses in the range.

To configure named address ranges in a routing instance, configure the address-assignment statements in the [edit routing-instances] hierarchy level.

Configuring Static Address Assignments

You can optionally create a static IPv4 address binding by reserving a specific address for a particular client. The address is removed from the address-assignment pool so that it is not assigned to another client. When you reserve an address, you identify the client host and create a binding between the client MAC address and the assigned IP address.

This example is tested on SRX300, SRX320, SRX340, SRX345, SRX1500, and SRX550M devices.

To configure a static IPv4 address binding:

  1. Specify the name of the IPv4 address-assignment pool containing the IP address you want to reserve for the client.
  2. Specify the name of the client for the static binding, the client MAC address, and the IP address to reserve for the client. This configuration specifies that the client with MAC address 01:03:05:07:09:0b is always assigned IP address 192.168.10.2.
Note:

To configure static binding for an IPv4 address in a routing instance, configure the address-assignment statements in the [edit routing-instances] hierarchy.

Configuring Address-Assignment Pool Linking

Address-assignment pool linking enables you to specify a secondary address pool for the device to use when the primary address-assignment pool is fully allocated. When the primary pool has no available addresses remaining, the device automatically switches over to the linked secondary pool and begins allocating addresses from that pool. The device uses a secondary pool only when the primary address-assignment pool is fully allocated.

To link a primary address-assignment pool named pool-1 to a secondary pool named pool-2, use the following option:

Configuring DHCP Client-Specific Attributes for Address-Assignment Pools

You use the address-assignment pool feature to include application-specific attributes when clients obtain an address. The client application, such as DHCP, uses the attributes to determine how addresses are assigned and to provide optional application-specific characteristics to the client. For example, the DHCP application might specify that a client that matches certain prerequisite information is dynamically assigned an address from a particular named range. Based on which named range is used, DHCP specifies additional DHCP attributes such as the boot file that the client uses, the DNS server, and the maximum lease time.

Note:

This feature is supported on SRX300, SRX320, SRX340, SRX345, SRX550M, and SRX1500 devices.

You use the dhcp-attributes statement to configure DHCP client-specific attributes for address-assignment pools.

To configure address-assignment pool attributes for DHCP clients:

  1. Specify the name of the address-assignment pool.
  2. Configure optional DHCP client attributes.
Note:

To configure DHCP client-specific attributes in a routing instance, configure the dhcp-attributes statements in the [edit routing-instances] hierarchy.

Related Documentation

Release History Table
Release
Description
18.1R1
Starting in Junos OS Release 18.1R1, search mechanism for an available address proceeds through a chain of linked pools. This behavior enables the DHCP to search addresses contiguously.
18.1R1
Starting in Junos OS Release 18.1R1, you can exclude a specified address or range of consecutive addresses to prevent them from being allocated from an address pool.