Configuration of the
Cisco IOS DHCP Server is simple and requires only a few steps:
- Define the DHCP database agent, an external FTP, TFTP, or RCP
host storing the DHCP bindings database. Several DHCP database
agents may be defined.
- Define and assign names to the DHCP address pool(s) from which IP
addresses are allocated to clients. This includes defining the range
of addresses in the pools, subnet masks, and address exclusions.
- On a per-address pool basis, specify DHCP options for clients
where necessary, including:
- default boot image name
- default router(s)
- default DNS server(s)
- DNS domain name
- IP address lease period (days, hours, minutes)
- NetBIOS name server
- NetBIOS node type (b, p, m, or h)
- Configure any required DHCP Forwarding functionality Intelligent
DHCP Relay and Secondary Address Pools.
With the introduction of Easy IP Phase 2, Cisco IOS software also
supports Intelligent DHCP Relay functionality. Generally speaking, a
DHCP Relay Agent is any host that forwards DHCP packets between
clients and servers. A DHCP relay agent enables the client and
server to reside on separate subnets. If the Cisco IOS DHCP Server
cannot satisfy a DHCP request from its own database, it can forward
the DHCP request to one or more secondary DHCP servers defined by
the network administrator via standard Cisco IOS "ip
helper-address" functionality (ip helper-address will be covered
in later in this course).
If the DHCP relay agent sees several DHCP retransmissions, it
assumes that the DHCP server is not responding because of an
exhausted address pool. The Intelligent DHCP Relay changes the
"giaddr" field (the "gateway ID" field, or IP
relay agent IP address, as described in RFC 2131) when it sees
client DHCP request retransmissions, forcing the downstream DHCP
Server to attempt to allocate an IP address from a different pool.4
This makes it possible for the client to boot with an IP address
from a secondary address pool if the primary pool is depleted.
Regular, "non-intelligent" DHCP Relays use the same giaddr field when forwarding client DHCP retransmissions,
making it impossible for the client to boot if the primary address
pool is depleted. The Cisco IOS DHCP Server itself enables
allocation of addresses from secondary address pools. Here we assume
that the router is configured with secondary IP addresses. If the
client is directly connected, the Cisco IOS DHCP Server attempts to
allocate an address from the primary pools. If no free addresses are
available in the primary pools, the server automatically allocates
an address from a secondary pool. As a result, clients can boot even
if all primary addresses are allocated. If all address pools are
depleted, the Cisco IOS DHCP Server silently drops the request.
Unique Address Allocation
In order to prevent assigning to clients addresses that may
already be in use, the Cisco IOS DHCP Server issues ICMP echo
requests to pool addresses before assigning them to clients. Although configurable, the default number of pings used
to check for potential IP address conflicts is 2. DHCP address pools
are stored in non-volatile RAM (NVRAM). There is no limit on the
number of configurable address pools.
|
|