Operation Manual – DHCP
H3C S3100 Series Ethernet Switches
Chapter 2 DHCP Snooping Configuration
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forwarding the packet, or will directly forward the packet if the packet does not contain
the Option 82 field.
2.1.5 Overview of IP Filtering
A denial-of-service (DoS) attack means an attempt of an attacker sending a large
number of forged address requests with different source IP addresses to the server so
that the network cannot work normally. The specific effects are as follows:
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The resources on the server are exhausted, so the server does not respond to
other requests.
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After receiving such type of packets, a switch needs to send them to the CPU for
processing. Too many request packets cause high CPU usage rate. As a result,
the CPU cannot work normally.
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The switch can filter invalid IP packets through the DHCP-snooping table and IP
static binding table.
I. DHCP-snooping table
After DHCP snooping is enabled on a switch, a DHCP-snooping table is generated. It is
used to record IP addresses obtained from the DHCP server, MAC addresses, the
number of the port through which a client is connected to the DHCP-snooping-enabled
device, and the number of the VLAN to which the port belongs to. These records are
saved as entries in the DHCP-snooping table.
II. IP static binding table
The DHCP-snooping table only records information about clients that obtains IP
address dynamically through DHCP. If a fixed IP address is configured for a client, the
IP address and MAC address of the client cannot be recorded in the DHCP-snooping
table. Consequently, this client cannot pass the IP filtering of the DHCP-snooping table,
thus it cannot access external networks.
To solve this problem, the switch supports the configuration of static binding table
entries, that is, the binding relationship between IP address, MAC address, and the port
connecting to the client, so that packets of the client can be correctly forwarded.
III. IP filtering
The switch can filter IP packets in the following two modes:
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Filtering the source IP address in a packet. If the source IP address and the
number of the port that receives the packet are consistent with entries in the
DHCP-snooping table or static binding table, the switch regards the packet as a
valid packet and forwards it; otherwise, the switch drops it directly.
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Filtering the source IP address and the source MAC address in a packet. If the
source IP address and source MAC address in the packet, and the number of the
port that receives the packet are consistent with entries in the DHCP-snooping
