Chapter 24 IP Source Guard

24.1.1.3 DHCP Relay Option 82 Information

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 33 on page 259 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 33 on page 259).

24.1.1.4 Configuring DHCP Snooping

Follow these steps to configure DHCP snooping on the Switch.

1Enable DHCP snooping on the Switch.

2Enable DHCP snooping on each VLAN, and configure DHCP relay option 82.

3Configure trusted and untrusted ports, and specify the maximum number of DHCP packets that each port can receive per second.

4Configure static bindings.

24.1.2ARP Inspection Overview

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.

Figure 99 Example: Man-in-the-middle Attack

AB

X

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.

•It pretends to be computer B and sends a message to computer A.

As a result, all the communication between computer A and computer B passes through computer X. Computer X can read and alter the information passed between them.

 

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