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 first attempts to match the giaddr to a local pool network address. If it does not find a match, the router attempts to match the received interface IP address to a local pool network address.
- 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 server. 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.
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 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 following precedence:
- If AAA specifies an IP address, the DHCP local server finds the address pool containing the address, then allocates that address.
- If AAA specifies an address pool name, the local server finds the pool with the matching name and allocates an address from that pool.
- The local server finds the address pool whose name matches the client's domain.
- The local server finds the address pool whose domain name matches the client's domain.
- The local server finds the address pool whose IP network matches the client's DHCP giaddr.
- The local server finds the address pool whose interface matches the interface on which the client's DHCP request was received.
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.