Configuring Address-Assignment Pool Linking

 

Address-assignment pool linking enables you to specify a secondary address pool for the router to use when the matching or primary address-assignment pool is fully allocated. You can create a chain of multiple linked pools. For example, you can link Pool A to Pool B, and link Pool B to Pool C. You can link any number of pools serially in a chain, but you cannot create multiple links to or from the same pool. For example, you cannot create links from Pool A to both Pool B and Pool C. Similarly, Pool C cannot be linked from both Pool A and Pool B. An additional consideration is that all address pools in a chain must be of the same family type, IPv4 or IPv6.

When the address pool that matches the subscribers has no available addresses, the router automatically switches over to the linked pool and allocates addresses from that pool. The router uses a linked pool only when the matching address-assignment pool is fully allocated.

Starting in Junos OS Release 18.1, the behavior changes for how to find and allocate a free address in a chain of address pools. You can configure linked pools to be searched in one of two ways:

  • Contiguous address allocation—Default behavior. All addresses in each range of a pool are searched. The search starts in the matched pool, then moves to the first pool in the chain and, if necessary, continues through each linked pool successively to the last pool in the chain. In each pool, all addresses in all ranges are searched for a free address. This method enables addresses to be assigned contiguously; each pool has to be full before another pool is searched.

  • Noncontiguous (aggregated) address allocation—Behavior when linked-pool-aggregation is configured. Initially, only certain addresses (from nextAddress to highAddress) are searched in each range of the matched pool. The same search is performed in the linked pool, if necessary, and continues through each successive linked pool through the last pool in the chain.

    The search then restarts at the first pool in the chain (not necessarily the matched pool). This time, all addresses in all ranges are searched, in all pools through the end of the chain.

Including the linked-pool-aggregation statement might be desirable if you configure your RADIUS server to use the IP address alone to identify subscribers. Typically, subscribers are identified by the RADIUS server using the subscriber session ID and other criteria. If you use only the IP address, you might encounter the following issue with the default behavior when the linked-pool-aggregation statement is not configured. A subscriber can disconnect and that address can be assigned to the next subscriber. The Acct-Start message for the second subscriber might be sent before the Acct-Stop message is sent for the disconnected subscriber. When the Acct-Stop is received, the new subscriber, identified only by the IP address, can be disconnected.

You can avoid this situation by either including the linked-pool-aggregation statement or configuring your RADIUS server to use the subscriber session ID (instead of the IP address) for identification.

Before you begin, configure your address pools. See Address-Assignment Pool Configuration Overview.

To link an address-assignment pool to a secondary pool:

  1. Specify the names of the pools to be linked.
  2. (Optional) Configure searching to allow for noncontiguous address allocation.

For example, the following configuration links Pool_A to Pool_B and then links Pool_B to Pool_C.

Release History Table
Release
Description
Starting in Junos OS Release 18.1, the behavior changes for how to find and allocate a free address in a chain of address pools.