ZyXEL Communications XGS-4526 User Manual

 Chapter 26 IP Source Guard

XGS-4526/4528F/4728F User’s Guide

You can configure the name and location of the file on the external TFTP server. 
The file has the following format:

Figure 115   DHCP Snooping Database File Format

The <initial-checksum> helps distinguish between the bindings in the latest 
update and the bindings from previous updates. Each binding consists of 72 bytes, 
a space, and another checksum that is used to validate the binding when it is 
read. If the calculated checksum is not equal to the checksum in the file, that 
binding and all others after it are ignored.

The Switch can add information to DHCP requests that it does not discard. This 
provides the DHCP server more information about the source of the requests. The 
Switch can add the following information:

• Slot ID (1 byte), port ID (1 byte), and source VLAN ID (2 bytes)
• System name (up to 32 bytes)

This information is stored in an Agent Information field in the option 82 field of the 
DHCP headers of client DHCP request frames. See 

 for 

more information about DHCP relay option 82.

When the DHCP server responds, the Switch removes the information in the Agent 
Information field before forwarding the response to the original source.

You can configure this setting for each source VLAN. This setting is independent of 
the DHCP relay settings (

Follow these steps to configure DHCP snooping on the Switch.

Enable DHCP snooping on the Switch.

Enable DHCP snooping on each VLAN, and configure DHCP relay option 82.

<initial-checksum>

TYPE DHCP-SNOOPING

VERSION 1

BEGIN

<binding-1> <checksum-1>

<binding-2> <checksum-1-2>

...

...

<binding-n> <checksum-1-2-..-n>

END