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Chapter 5 Tutorials
GS1910/XGS1910 Series User’s Guide
46
If you want to use dynamic bindings to filter unauthorized ARP packets (typical implementation),
you have to enable DHCP snooping before you enable ARP inspection.
Trusted vs. Untrusted Ports
Every port is either a trusted port or an untrusted port for DHCP snooping.
Trusted ports are connected to DHCP servers or other switches. 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.
In the following example, you only want DHCP server A connected to port 5 to assign IP addresses
to all devices in VLAN 100. Create a VLAN containing ports 5, 6 and 7. Connect a computer (M) to
the Switch’s port which is not in VLAN 100.
The settings in this tutorial are as the following.
1 Access the Switch through http://192.168.1.1. Log into the Switch by entering the username
(default: admin) and password (default: 1234).
2 See Section 5.4 on page 38 for how to create a VLAN and configure ports to join the VLAN.
Table 5 Settings in this Tutorial
HOST
PORT
CONNECTED
VLAN PVID
DHCP SNOOPING PORT
TRUSTED
DHCP Server (A) 5 1 and 100 100 Yes
DHCP Client (B) 6 1 and 100 100 No
DHCP Client (C) 7 1 and 100 100 No
A
B
M
C
VLAN 100