21-6
Catalyst 3750 Switch Software Configuration Guide
78-16180-02
Chapter 21 Configuring DHCP Features and IP Source Guard
Understanding DHCP Features
When a switch learns of new bindings or when it loses bindings, the switch updates the entries in the
database and in the binding file. The frequency at which database and file are updated is based on a
configurable delay, and the updates are batched. If the database and file are 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 that has the 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:
3ebe1518
TYPE DHCP-SNOOPING
VERSION 1
BEGIN
1.1.1.1 512 0001.0001.0005 3EBE2881 Gi1/0/1 e5e1e733
1.1.1.1 512 0001.0001.0002 3EBE2881 Gi1/0/1 4b3486ec
1.1.1.1 1536 0001.0001.0004 3EBE2881 Gi1/0/1 f0e02872
1.1.1.1 1024 0001.0001.0003 3EBE2881 Gi1/0/1 ac41adf9
1.1.1.1 1 0001.0001.0001 3EBE2881 Gi1/0/1 34b3273e
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.
When a stack merge occurs, all DHCP snooping bindings in the stack master are lost if it is no longer
the stack master. With a stack partition, the existing stack master is unchanged, and the bindings
belonging to the partitioned switches age out. The new master of the partitioned stack begins processing
the new incoming DHCP packets. For more information about switch stacks, see Chapter 5, “Managing
Switch Stacks.”
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