ZyXEL Communications VES1724-56 User Manual page 250

Chapter 26 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.
Note: If DHCP is enabled and there are no trusted ports, DHCP requests will not succeed.
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.
26.1.1.2 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 147 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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VES1724-56 User's Guide