ZyXEL Communications GS2200-24 User Manual page 236

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.
26.12.1.2 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 114 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
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GS2200-24 User's Guide