ZyXEL Communications MES3500-24S User Manual page 236

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: 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.
27.1.1.2 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 138 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.
27.1.1.3 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)
Chapter 27 IP Source Guard
MES3500-24S/MGS3520 Series User's Guide
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