Figure 86 Dhcp Snooping Database File Format - ZyXEL Communications Dimension ES-2048 User Manual

Chapter 21 IP Source Guard
Trusted ports are connected to DHCP servers or other switches. The Switch discards DHCP
packets from trusted ports only if the rate at which DHCP packets arrive is too high. The
Switch learns dynamic bindings from trusted ports.
The Switch will drop all DHCP requests if you enable DHCP snooping and
there are no trusted ports.
Untrusted ports are connected to subscribers. The Switch discards DHCP packets from
untrusted ports in the following situations:
• The packet is a DHCP server packet (for example, OFFER, ACK, or NACK).
• The source MAC address and source IP address in the packet do not match any of the
current bindings.
• The packet is a RELEASE or DECLINE packet, and the source MAC address and source
port do not match any of the current bindings.
• The rate at which DHCP packets arrive is too high.
21.1.1.2 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 86 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.
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ES-2048 User's Guide