Introduction To Ip Filtering - H3C S5600 Series Operation Manual

Operation Manual – DHCP
H3C S5600 Series Ethernet Switches
Note:
The circuit ID and remote ID sub-options in Option 82, which can be configured
simultaneously or separately, are independent of each other in terms of configuration
sequence.
When the DHCP snooping device receives a DHCP response packet from the DHCP
server, the DHCP snooping device will delete the Option 82 field, if contained, before
forwarding the packet, or will directly forward the packet if the packet does not contain
the Option 82 field.

4.1.3 Introduction to 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:
The resources on the server are exhausted, so the server does not respond to
other requests.
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.
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.
Chapter 4 DHCP Snooping Configuration
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