Cisco IE 3000, Руководство по настройке программного обеспечения

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23-5
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 23 Configuring DHCP Features and IP Source Guard
Understanding DHCP Snooping
In the port field of the circuit-ID suboption, the port numbers start at 3. For example, on a switch with
eight 10/100 ports and small form-factor pluggable (SFP) module slots, port 3 is the Fast Ethernet 1/1
port, port 4 is the Fast Ethernet 1/2 port, and so forth. Port 11 is the SFP module slot 1/1, and so forth.
Figure 23-2 shows the packet formats for the remote-ID suboption and the circuit-ID suboption. The
switch uses the packet formats when you globally enable DHCP snooping and enter the ip dhcp
snooping information option global configuration command.
Figure 23-2 Suboption Packet Formats
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 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 database agent. If the
agent is disabled, dynamic ARP inspection or IP source guard 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 connectivity, but DHCP snooping might
not prevent DHCP spoofing attacks.
When reloading, the switch reads the binding file to build the DHCP snooping 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
S uboption
type
Circuit ID Suboption Frame Format
Remote ID Suboption Frame Format
6 byte s
MAC a ddre ss
1 byte 1 byte 1 byte
S uboption
type
1 byte
Length Length
Remote
ID type
1 byte 1 byte 1 byte 1 byte
116
3
00
4 0 6 1
6 0 8 2
Module Port
1 byte 1 byte 2 byte s
VLAN