Cisco Systems 3560 Manual Do Utilizador

For information about the DHCP client, see the “Configuring DHCP” section of the “IP Addressing and 
Services” section of the Cisco IOS IP Configuration Guide, Release 12.2 from the Cisco.com page under 
Documentation > Cisco IOS Software > 12.2 Mainline > Configuration Guides.

The DHCP server assigns IP addresses from specified address pools on a switch or router to DHCP 
clients and manages them. If the DHCP server cannot give the DHCP client the requested configuration 
parameters from its database, it forwards the request to one or more secondary DHCP servers defined 
by the network administrator. 

A DHCP relay agent is a Layer 3 device that forwards DHCP packets between clients and servers. Relay 
agents forward requests and replies between clients and servers when they are not on the same physical 
subnet. Relay agent forwarding is different from the normal Layer 2 forwarding, in which IP datagrams 
are switched transparently between networks. Relay agents receive DHCP messages and generate new 
DHCP messages to send on output interfaces.

DHCP snooping is a DHCP security feature that provides network security by filtering untrusted DHCP 
messages and by building and maintaining a DHCP snooping binding database, also referred to as a 
DHCP snooping binding table.

DHCP snooping acts like a firewall between untrusted hosts and DHCP servers. You use DHCP 
snooping to differentiate between untrusted interfaces connected to the end user and trusted interfaces 
connected to the DHCP server or another switch. 

For DHCP snooping to function properly, all DHCP servers must be connected to the switch through 
trusted interfaces.

An untrusted DHCP message is a message that is received from outside the network or firewall. When 
you use DHCP snooping in a service-provider environment, an untrusted message is sent from a device 
that is not in the service-provider network, such as a customer’s switch. Messages from unknown 
devices are untrusted because they can be sources of traffic attacks.

The DHCP snooping binding database has the MAC address, the IP address, the lease time, the binding 
type, the VLAN number, and the interface information that corresponds to the local untrusted interfaces 
of a switch. It does not have information regarding hosts interconnected with a trusted interface. 

In a service-provider network, a trusted interface is connected to a port on a device in the same network. 
An untrusted interface is connected to an untrusted interface in the network or to an interface on a device 
that is not in the network.