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Chapter 25 IP Source Guard

GS2200-8/24 User’s Guide

212

25.10.1.2 DHCP Snooping Database

The Switch stores the binding table in volatile memory. If the Switch restarts, it loads static

bindings from permanent memory but loses the dynamic bindings, in which case the devices in the

network have to send DHCP requests again. As a result, it is recommended you configure the DHCP

snooping database.

The DHCP snooping database maintains the dynamic bindings for DHCP snooping and ARP

inspection in a file on an external TFTP server. If you set up the DHCP snooping database, the

Switch can reload the dynamic bindings from the DHCP snooping database after the Switch

restarts.

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

following format:

Figure 132 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.

25.10.1.3 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 32 on page 244 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 32 on page 244).

<initial-checksum>

TYPE DHCP-SNOOPING

VERSION 1

BEGIN

<binding-1> <checksum-1>

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

...

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

END