25-6
Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 25 Configuring DHCP
Information About Configuring DHCP
Figure 25-3 User-Configured Suboption Packet Formats
Cisco IOS DHCP Server Database
During the DHCP-based autoconfiguration process, the designated DHCP server uses the Cisco IOS
DHCP server database. It has IP addresses, address bindings, and configuration parameters, such as the
boot file.
An address binding is a mapping between an IP address and a MAC address of a ho st in the Cisco IOS
DHCP server database. You can manually assign the client IP address, or the DHCP server can allocate
an IP address from a DHCP address pool.
DHCP Snooping Binding Database
When DHCP snooping is enabled, the switch uses the DHCP snoopi ng binding database to store
information about untrusted interfaces. The database can have up to 8192 bindings.
Each database entry (binding) has an IP address, an associated MAC address, the lease time (in
hexadecimal format), the interface to which the binding applies, and the VLAN to which the interface
belongs. The database agent stores the bindings in a file at a configured location. At the end of each entry
is a checksum that accounts for all the bytes from the start of the file through all the bytes associated
with the entry. Each entry is 72 bytes, followed by a space and then the checksum value.
To keep the bindings when the switch reloads, you must use the DHCP snooping data base agent. If the
agent is disabled, dynamic ARP inspection or IP source g uard is enabled, and the DHCP snooping
binding database has dynamic bindings, the switch loses its connectivity. If the agent is disabled and only
DHCP snooping is enabled, the switch does not lose its connec tivity, but DHCP snooping might not
prevent DHCP spoofing attacks.
When reloading, the switch reads the binding file to build the DHCP sn ooping binding database. The
switch updates the file when the database changes.
Length Length
Circuit
ID type
Suboption
type

Circuit ID Suboption Frame Format (for user-configured string):

Remote ID Suboption Frame Format (for user-configured string):

Suboption
type
Length Length
Remote
ID type
145774
N1N+2 1
N1N+2 2
ASCII Circuit ID string
1 byte 1 byte 1 byte N bytes (N = 3-63)
ASCII Remote ID string or hostname
N bytes (N = 1-63)
1 byte
1 byte 1 byte 1 byte1 byte