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Chapter 26 IP Source Guard
XGS-4526/4528F/4728F User’s Guide
261
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.
26.1.1.3 DHCP Relay Option 82 Information
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 Chapter 41 on page 361 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 (Chapter 41 on page 361).
26.1.1.4 Configuring DHCP Snooping
Follow these steps to configure DHCP snooping on the Switch.
1 Enable DHCP snooping on the Switch.
2 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