ZyXEL Communications OLT2406 User Manual page 242

27.10.1.1 Trusted vs. Untrusted Ports
Every port is either a trusted port or an untrusted port for DHCP snooping. This setting is independent of
the trusted/untrusted setting for ARP inspection. You can also specify the maximum number for DHCP
packets that each port (trusted or untrusted) can receive each second.
Trusted ports are connected to DHCP servers or other switches. The OLT discards DHCP packets from
trusted ports only if the rate at which DHCP packets arrive is too high. The OLT 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 OLT 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.
27.10.1.2 DHCP Snooping Database
The OLT stores the binding table in volatile memory. If the OLT 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 OLT can reload the
dynamic bindings from the DHCP snooping database after the OLT restarts.
You can configure the name and location of the file on the external TFTP server. The file has the following
format:
Figure 145 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.
Chapter 27 IP Source Guard
OLT2406 User's Guide
242