ZyXEL Communications XS3800-28, User Manual, Page: 618 / 789

Chapter 83 DHCP Snooping
XS3800-28 User’s Guide
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83.7 Technical Reference
This section provides technical background information on the topics discussed in this chapter.
83.7.1 DHCP Snooping Overview
Use DHCP snooping to filter unauthorized DHCP packets on the network and to build the binding table
dynamically. This can prevent clients from getting IP addresses from unauthorized DHCP servers.
83.7.1.1 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 or 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 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.
Option 82 Profile Select a pre-defined DHCP option 82 profile that the Switch applies to the specified ports in
this VLAN. The Switch adds the information (such as slot number, port number, VLAN ID and/
or system name) specified in the profile to DHCP requests that it broadcasts to the DHCP
VLAN, if specified, or VLAN. You can specify the DHCP VLAN in the
SECURITY > IPv4 Source
Guard > DHCP Snooping > DHCP Snp. Setup screen.
Note: The profile you select here has priority over the one you select in the
SECURITY > IPv4 Source Guard > DHCP Snooping > DHCP Snp. VLAN Setup
screen.
Apply Click Apply to save your changes to the Switch’s run-time memory. The Switch loses these
changes if it is turned off or loses power, so use the Save link on the top navigation panel to
save your changes to the non-volatile memory when you are done configuring.
Clear Click Clear to clear the fields to the factory defaults.
Cancel Click Cancel to not save the configuration you make and return to the last screen.
Table 331 SECURITY > IPv4 Source Guard > DHCP Snooping > DHCP Snp. VLAN Port Setup > Add/Edit
LABEL DESCRIPTION