Cisco WS-C3750-48PS-S User Manual

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Catalyst 3750 Switch Software Configuration Guide
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Chapter 21 Configuring DHCP Features and IP Source Guard
Understanding DHCP Features
Figure 21-2 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 host 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. For more information about manual and automatic address
bindings, refer to the “Configuring DHCP” chapter of the Cisco IOS IP Configuration Guide,
Release 12.2.
DHCP Snooping Binding Database
When DHCP snooping is enabled, the switch uses the DHCP snooping binding database to store
information about untrusted interfaces. The database can have up to 512 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. A checksum value, the end of each entry, is the number of bytes from the start of the file to end
of 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 database agent. If the
agent is disabled, the switch loses its DHCP snooping bindings and its connectivity when it reloads. The
switch also loses connectivity.
The database agent stores the bindings in a file at a configured location. When reloading, the switch
reads the binding file to build the DHCP snooping binding database. The switch keeps the file current
by updating it when the database changes.
Length Length
Circuit
ID type
Suboption
type
Circuit ID Suboption Frame Format
Remote ID Suboption Frame Format
6 bytes
MAC address
1 byte 1 byte 1 byte
Suboption
type
1 byte
Length Length
Remote
ID type
1 byte 1 byte 1 byte1 byte
116300
4061
6082
Module Port
1 byte 1 byte2 bytes
VLAN

Cisco WS-C3750-48PS-S Specifications

General

Brand	Cisco
Model	WS-C3750-48PS-S
Category	Switch
Language	English