ZyXEL Communications OLT2406, User Manual

Page: 549 / 766

Chapter 77 IP Source Guard
OLT2406 User’s Guide
549
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: The OLT will drop all DHCP requests if you enable DHCP snooping and there are no
trusted ports.
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.
77.1.1.2 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 272 DHCP Snooping Database File Format
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.
77.1.1.3 DHCP Relay Option 82 Information
The OLT 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 OLT can add the following information:
• Slot ID (One byte), port ID (One byte), and source VLAN ID (Two bytes)
<initial-checksum>
TYPE DHCP-SNOOPING
VERSION 1
BEGIN
<binding-1> <checksum-1>
<binding-2> <checksum-1-2>
...
...
<binding-n> <checksum-1-2-..-n>
END