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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 24 Configuring DHCP Features and IP Source Guard Understanding DHCP Features
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.
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 Gi0/4 21ae5fbb
192.1.168.3 3 0003.44d6.c52f 2BB648EB Gi0/4 1bdb223f
192.1.168.2 3 0003.47d9.c8f1 2BB648AB Gi0/4 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.
DHCP Snooping and Switch Stacks
DHCP snooping is managed on the stack master. When a new switch joins the stack, the switch receives
the DHCP snooping configuration from the stack master. When a member leaves the stack, all DHCP
snooping address bindings associated with the switch age out.
All snooping statistics are generated on the stack master. If a new stack master is elected, the statistics
counters reset.