Local Address Servers Configuration Overview

The local address server allocates IP addresses from a pool of addresses stored locally on the router. You can optionally configure shared local address pools to obtain addresses from a DHCP local address pool that is in the same virtual router. Addresses are provided automatically to client sessions requiring an IP address from a virtual router that is configured to use a local address pool.

A local address server is defined in the context of a virtual router. You create a local address server when you configure the first local pool. Local address servers exist as long as the virtual router exists or until you remove them by deleting all configured pools.

Figure 1 illustrates the local address pool hierarchy. Multiple local address server instances, one per virtual router. can exist. Each local address server can have one or more local address pools. Each pool can contain a number of IP addresses that are available for allocation and used by clients, such as PPP sessions.

Figure 1: Local Address Pool Hierarchy

Local Address Pool Hierarchy

The following sections describe local address servers:

Local Address Pool Ranges

As shown in Figure 1, each local address pool is named and contains ranges of sequentially ordered IP addresses. These addresses are allocated when the AAA server makes a request for an IP address.

If a local address pool range is exhausted, the next range of addresses is used. If all pool ranges are exhausted, you can configure a new range to extend or supplement the existing range of addresses, or you can create a new pool. The newly created pool range is then used for future address allocation. If addresses allocated from the first pool range are released, then subsequent requests for addresses are taken from the first pool range.

Addresses are assigned sequentially from a range within a pool. If a range has no addresses available, the next range within that pool is used. If a pool has no addresses available, the next configured pool is used, unless a specific pool is indicated.

Note: The AAA server does not allocate Class D and Class E addresses to the subscribers from the local pool, even if they are included in the local address pool range.

Local Address Pool Aliases

An alias is an alternate name for an existing local address pool. It comprises an alias name and a pool name.

When the AAA server requests an IP address from a specific local address pool, the local address server first verifies whether an alias exists for the requested pool. If an alias exists, the IP address is allocated from the pool specified by the alias. If no alias exists, the IP address is allocated from the pool originally specified in the request.

The use of aliases simplifies management of subscribers. For example, you can use an alias to migrate subscribers from one local address pool to another. Instead of having to modify countless subscriber records on the AAA server, you create an alias to make the configuration change.

Shared Local Address Pools

Typically, the local address server allocates IP addresses from a pool of addresses that is stored locally on the router. However, shared local address pools enable a local address server to hand out addresses that are allocated from DHCP local server address pools within the same virtual router. The addresses are configured and managed within DHCP. Therefore, thresholds are not configured on the shared pool, but are instead managed by the referenced DHCP local server pool.

A shared local address pool references one DHCP address pool. The shared local address pool can then obtain addresses from the referenced DHCP address pool and from any DHCP address pools that are linked to the referenced DHCP address pool.

Figure 2 illustrates a shared local address pool environment that includes four linked DHCP address pools. In the figure, both Shared_LAS_Pool_A and Shared_LAS_Pool_B reference DHCP_Pool_1, and can therefore obtain addresses from all four DHCP address pools. Shared_LAS_Pool_C references DHCP_Pool_3 and can get addresses from DHCP_Pool_3 and DHCP_Pool_4.

Figure 2: Shared Local Address Pools

Shared Local Address Pools

When the local address server requests an address from a shared address pool, the address is returned from the referenced DHCP pool or a subsequent linked pool. If no address is available, DHCP notifies the local address server and the search is ended.

Keep the following guidelines in mind when using shared local address pools:

The following commands create the shared address pools in Figure 2:

host1(config)#ip local shared-pool Shared_LAS_Pool_A DHCP_Pool_1 host1(config)#ip local shared-pool Shared_LAS_Pool_B DHCP_Pool_1 host1(config)#ip local shared-pool Shared_LAS_Pool_C DHCP_Pool_3

SNMP Thresholds

A local address pool has SNMP thresholds associated with it that enable the local address server to signal SNMP traps when certain conditions exist. These thresholds include high utilization threshold and abated utilization threshold. If the outstanding addresses of a pool or a pool group exceed the high utilization threshold and the SNMP trap signaling is enabled, SNMP is notified. Likewise, when a pool’s utilization drops below the abated utilization threshold, SNMP is notified.

A local address pool can be linked to a second local address pool so that when the first pool utilization reaches 100%, the DHCP local server uses the second pool. For generation of SNMP traps, the utilization of addresses is calculated for all the pools that are in the linked pools and they are collectively considered as an aggregated pool group.

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