Chapter 26 IP Source Guard
GS1920v2 Series User’s Guide
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26.11.1.3 DHC P Re la y O ptio n 82 Info rm a tio n
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)
• System name (up to 32 bytes)
This information is stored in an Agent Information field in the option 82 field of the DHCP headers of client
DHCP request frames. See
Chapter 37 on page 323
for more information about DHCP relay option 82.
When the DHCP server responds, the Switch removes the information in the Agent Information field
before forwarding the response to the original source.
You can configure this setting for each source VLAN. This setting is independent of the DHCP relay
settings (
Chapter 37 on page 323
).
26.11.1.4 C o nfig uring DHC P Sno o ping
Follow these steps to configure DHCP snooping on the Switch.
1
Enable DHCP snooping on the Switch.
2
Enable DHCP snooping on each VLAN, and configure DHCP relay option 82.
3
Configure trusted and untrusted ports, and specify the maximum number of DHCP packets that each
port can receive per second.
4
Configure static bindings.
26.11.2 ARP Inspe c tio n O ve rvie w
Use ARP inspection to filter unauthorized ARP packets on the network. This can prevent many kinds of
man-in-the-middle attacks, such as the one in the following example.
Fig ure 183
Example: Man-in-the-middle Attack
In this example, computer
B
tries to establish a connection with computer
A
. Computer
X
is in the same
broadcast domain as computer
A
and intercepts the ARP request for computer
A
. Then, computer
X
does the following things:
• It pretends to be computer
A
and responds to computer
B
.
