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DHCP Snooping
112
16.3 DHCP-Snooping Security
Configuration
16.3.1 Configure max clients number
A private DHCP server on a network also answers IP address request packets and assigns
IP addresses to DHCP clients. However, the IP addresses they assigned may conflict with
those of other hosts. As a result, users cannot normally access networks. This kind of DHCP
servers are known as private DHCP servers. Therefore, administrators can:
Restrict the DHCP-Client number connected to switch port. So only the clients connected
to the same port with the attacker will suffer the attack.
Restrict the DHCP-Client number in specified VLAN. So only the clients in the same VLAN
with the attacker will suffer the attack.
This function should be work with DHCP-Snooping. Perform following commands in
interface configuration mode.
Table 16-2 Configure max clients number
Operation
Command
Description
Configure max DHCP-Client
number connected to switch
port
dhcp-snooping max-clients
<0-2048>
By default, the max
DHCP-Client number
connected to switch port is
2048.
Enter VLAN mode
vlan vlan_list
Configure max DHCP-Client
number in specified VLAN.
dhcp-snooping max-clients
<0-2048>
By default, the max
DHCP-Client number in
specified VLAN is 2048.
16.3.2 Configure IP-Source-Guard
IP Source Guard provides source IP address filtering on a Layer 2 port to prevent a
malicious host from impersonating a legitimate host by assuming the legitimate host's IP
address. The feature uses dynamic DHCP snooping and static IP source binding to match IP
addresses to hosts on untrusted Layer 2 access ports. When using IP-Source-Guard, pay
attention:
DHCP-Snooping has been enabled
Use this function in Trust port
After enabling IP-Source-Guard, all traffic with that IP source address is permitted from
that trusted client. Traffic from other hosts is denied. This filtering limits a host's ability to
attack the network by claiming a neighbor host's IP address. The filtering info can be source
MAC, source IP and source port number.