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Configuring DHCP
Information About Configuring DHCP
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.
Default DHCP Snooping Settings
Table 46
Default DHCP Snooping Settings
Feature
DHCP server
DHCP relay agent
DHCP packet forwarding address
Checking the relay agent information
DHCP relay agent forwarding policy
DHCP snooping enabled globally
DHCP snooping information option
DHCP snooping option to accept packets on
untrusted input interfaces
DHCP snooping limit rate
Default Setting
Enabled in Cisco IOS software, requires configuration
Enabled
None configured
Enabled (invalid messages are
Replace the existing relay agent
Disabled
Enabled
Disabled
3
None configured
2
dropped)2. on page 395
information2. on page 395
394
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