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    Standalone Mode Overview

    In standalone mode, the DHCP local server operates as a basic DHCP server. Clients are not authenticated by default; however, you can optionally configure the DHCP local server to use AAA authentication for the incoming clients. The DHCP local server receives DHCP client requests for addresses, selects DHCP local pools from which to allocate addresses, distributes addresses to the clients, and maintains the resulting DHCP bindings in a server management table.

    Local Pool Selection and Address Allocation

    In standalone mode, the DHCP local server selects a pool to allocate an address for a client; the SRC software is never notified or queried. The process used depends on whether AAA authentication is configured.

    • If AAA authentication is not configured, the DHCP local server selects a pool by matching the local pool network address to the giaddr or the received interface IP address. The router compares the parameters with the local DHCP pools in the order presented in Table 1. When the router finds a match, it selects a pool based on the match and does not examine other parameters.

      Table 1: Local Pool Selection in Standalone Mode Without AAA Authentication

      Field

      How the DHCP Local Server Uses the Field

      Giaddr

      A giaddr, which indicates a client’s subnetwork, can be presented to the DHCP local server in the client DHCP REQUEST message. The giaddr field in the DHCP request message usually contains the IP address of a DHCP relay agent. The router attempts to match the giaddr address in the DHCP request message with the network address of a DHCP local pool. If it finds a match, the router uses the matching DHCP local pool.

      Received interface IP address

      The router uses the IP address of the interface on which the DHCP packet is being processed and attempts to match it with the network address of a DHCP local pool.

      After the router selects a DHCP local pool, the DHCP local server first tries to find a reserved IP address for the client in the selected pool. If no reserved address is available, the router attempts to allocate a client’s requested IP address. If the requested IP address is not available, the router allocates the next available address in the pool. If a grace period is configured for the pool, the router assigns the grace period to the allocated address. If no addresses are available in a pool, the DHCP local server attempts to allocate an address from the linked pool, if such a pool is configured.

    • If AAA authentication is configured (as described in Configuring AAA Authentication for DHCP Local Server Standalone Mode ) and the authentication is successful, the local server selects an IP address pool based on the order presented in Table 2. When the router finds a match, it selects a pool based on the match and does not examine other parameters.

      Table 2: Local Pool Selection in Standalone Mode with AAA Authentication

      Field

      How the DHCP Local Server Uses the Field

      Framed IP address

      The client’s RADIUS entry can be configured with a framed IP address, which the DHCP local server can get from the AAA server when the client is authenticated.

      If the AAA server specifies a framed IP address, the DHCP local server attempts to allocate the address pool that contains the framed IP address and allocates that address from the pool. If the framed IP address is not available, then the server allocates the next available address in the pool to the client.

      Pool name

      Each DHCP local pool has a pool name. The client’s RADIUS entry can also be configured with a pool name, which the DHCP local server can get from the AAA server when the client is authenticated. The AAA server must be configured to send RADIUS attributes to DHCP.

      If AAA specifies an address pool name, the local server finds the pool with the matching name and allocates an address from that pool.

      Domain name

      You can use a domain name as the name of a DHCP local pool. If RADIUS authenticates the client using a domain name, the DHCP local server receives the domain name from the AAA server.

      If the client’s domain name does not match the name of the DHCP local pool, the router attempts to match the client’s domain name to the domain name field within the pool.

      Giaddr

      A DHCP local pool is configured with a network address. A gateway IP address (giaddr), which indicates a client’s subnetwork, can be presented to the DHCP local server in the client’s DHCP request message. The giaddr field in the DHCP request message usually contains the IP address of a DHCP relay server. The router attempts to match the giaddr address in the DHCP request message with the network address of a DHCP local pool.

      Received interface IP address

      The router uses the IP address of the interface on which the DHCP packet is being processed and attempts to match it with the network address of a DHCP local pool. If the interface address matches with the IP address configured in the DHCP local address pool on the router, that pool is used to delegate the address to the client.

    Server Management Table

    For each client that makes requests of the DHCP local server, the router keeps an entry in the server management table. The entry defines client-specific information and state information. The router uses this table to identify clients when it receives subsequent messages and to maintain the state of each client within the DHCP protocol. In addition, the table contains information that may be transferred to and from the SRC software.

    Published: 2014-08-20