Cisco Systems 3750E manual 22-7

Chapter 22 Configuring DHCP Features and IP Source Guard

Understanding DHCP Features

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.

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-checksumentry 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 Gi1/0/4 21ae5fbb 192.1.168.3 3 0003.44d6.c52f 2BB648EB Gi1/0/4 1bdb223f 192.1.168.2 3 0003.47d9.c8f1 2BB648AB Gi1/0/4 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.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 22-7