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Catalyst 2960 Switch SoftwareConfiguration Guide
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Chapter19 Configuring DHCP Feat ures
Understanding DHCP Features
In the port field of the circuit ID suboption, the port numbers start at 3. For example, on a switch with
24 10/100 ports and small form-factor pluggable (SFP) module slots, port 3 is the Fast Ethernet 0/1 port,
port 4 is the Fast Ethernet 0/2 port, and so forth. Port 27 is the SFP module slot 0/1, and so forth.
Figure 19-2 shows the packet formats for the remote ID suboption and the circuit ID suboption. The
switch uses the packet formats when DHCP snooping is globally enabled and when the ip dhcp
snooping information option global configuration command is entered.
Figure19-2 Suboption Packet Formats
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 and only DHCP snooping is enabled, the switch does not lose its connectivity, 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.
When a switch learns of new bindings or when it loses bindings, the switch immediately updates the
entries in the database. The switch also updates the entries in the binding file. The frequency at which
the file is updated is based on a configurable delay, and the updates are batched. If the file is not updated
in a specified time (set by the write-delay and abort-timeout values), the update stops.
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