23-6
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 23 Configuring DHCP Features and IP Source Guard
Configuring DHCP Snooping
This is the format of the file with bindings:
<initial-checksum>
TYPE DHCP-SNOOPING
VERSION 1
BEGIN
<entry-1> <checksum-1>
<entry-2> <checksum-1-2>
...
...
<entry-n> <checksum-1-2-..-n>
END
Each entry in the file is tagged with a checksum value that the switch uses to verify the entries when it
reads the file. The initial-checksum entry on the first line distinguishes entries associated with the latest
file update from entries associated with a previous file update.
This is an example of a binding file:
2bb4c2a1
TYPE DHCP-SNOOPING
VERSION 1
BEGIN
192.1.168.1 3 0003.47d8.c91f 2BB6488E interface-id 21ae5fbb
192.1.168.3 3 0003.44d6.c52f 2BB648EB interface-id 1bdb223f
192.1.168.2 3 0003.47d9.c8f1 2BB648AB interface-id 584a38f0
END
When the switch starts and the calculated checksum value equals the stored checksum value, the switch
reads entries from the binding file and adds the bindings to its DHCP snooping binding database. The
switch ignores an entry when one of these situations occurs:
• The switch reads the entry and the calculated checksum value does not equal the stored checksum
value. The entry and the ones following it are ignored.
• An entry has an expired lease time (the switch might not remove a binding entry when the lease time
expires).
• The interface in the entry no longer exists on the system.
• The interface is a routed interface or a DHCP snooping-trusted interface.
Configuring DHCP Snooping
These sections contain this configuration information:
• Default DHCP Snooping Configuration, page 23-6
• DHCP Snooping Configuration Guidelines, page 23-7
• Configuring the DHCP Relay Agent, page 23-8
• Enabling DHCP Snooping and Option 82, page 23-9
• Enabling the DHCP Snooping Binding Database Agent, page 23-11
Default DHCP Snooping Configuration
Table 23-1 shows the default DHCP snooping configuration.
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