Chapter 26 IP Source Guard

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

<initial-checksum>

TYPE DHCP-SNOOPING VERSION 1

BEGIN

<binding-1> <checksum-1> <binding-2> <checksum-1-2>

...

...

<binding-n> <checksum-1-2-..-n>

END

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.

1Enable DHCP snooping on the Switch.

2Enable DHCP snooping on each VLAN, and configure DHCP relay option 82.

 

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XGS-4526/4528F/4728F User’s Guide