Configure DHCP for DSL Router Users

The CPE router can function as a Cisco IOS-based DHCP server for subscriber network hosts. Address pools are configured for each subnet to be serviced. The address of the Ethernet interface should be excluded from the address range defined for the DHCP server. This is also the case for any other statically assigned host addresses on the subscriber's network such as print servers. The Cisco IOS DHCP functionality has been enhanced to support centralized DHCP services and administration. The pool definition(s) can be imported from centralized servers if desired.

Configure Static Default Route on a DSL Router 119

Example 5-5 can be added to the CPE router configuration discussed up to this point to enable DHCP services for the subscriber network.

Example 5-5 DHCP Services Configuration

iP !

dhcp excluded-address 172.16.0

5.1 172.16.«

iP

dhcp pool PCLAN

import all

network 172.16.0.0 255.255.0.«

!

default-router 172.16.0.1

The dhcp excluded-address command specifies that no addresses in the defined range should be allocated. Because of this, the first address available for host allocation is 172.16.0.10. Technically, the 172.16.0.1 address need not be included in the exclusion because the local router already has this address assigned, but it was included for clarity's sake. The import all option will dynamically populate any DNS server, WINS server, or other options, such as TFTP server, into the database so that they can be provided to hosts on the subscriber network.

If multiple VLANs are defined, each VLAN interface will provide addresses from the pool that shares its IP subnet. When a router receives a DHCP request, it checks all configured DHCP pools for a network match. If a match is found, an address will be assigned from the appropriate pool. If no match is found, no DHCP offer is made. To service the request, the router would require an additional pool configuration matching the network in question. Alternatively, if no pool is sharing its subnet, an IP helper address must be configured to forward the DHCP request to the appropriate server or no address will be allocated.

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