Chapter 25 IP Source Guard
GS1920 Series User’s Guide
220
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 159
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 35 on page 281 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 35 on page 281).
25.10.1.4
Configuring DHCP Snooping
Follow these steps to configure DHCP snooping on the Switch.
<initial-checksum>
TYPE DHCP-SNOOPING
VERSION 1
BEGIN
<binding-1> <checksum-1>
<binding-2> <checksum-1-2>
...
...
<binding-n> <checksum-1-2-..-n>
END
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