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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
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
- 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
- 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
- If AAA specifies an IP address, the DHCP local server
finds the address pool containing the address, then allocates that
- If AAA specifies an address pool name, the local server
finds the pool with the matching name and allocates an address from
- 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.
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