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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 25 Configuring DHCP
Information About Configuring DHCP
DHCP Relay Agent
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 n ot 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
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.
Note 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 f rom 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 correspon ds 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.
When a switch receives a packet on an untrusted i nterface and the interface belongs to a VLAN in which
DHCP snooping is enabled, the switch compares the source MAC address and the DHCP client hardware
address. If the addresses match (the default), the switch forwards the packet. If the addresses do not
match, the switch drops the packet.
The switch drops a DHCP packet when one of these situations occurs:
• A packet from a DHCP server, such as a DHCPOFFER, DHCPACK, DHCPNAK, or
DHCPLEASEQUERY packet, is received from outside the network or firewall.
• A packet is received on an untrusted interface, and the source MAC address and the DHCP client
hardware address do not match.
• The switch receives a DHCPRELEASE or DHCPDECLINE broadcast message that has a MAC
address in the DHCP snooping binding database, but the interface infor mation in the binding
database does not match the interface on which the message was received.
• A DHCP relay agent forwards a DHCP packet that includes a relay-agent IP address that is not
0.0.0.0, or the relay agent forwards a packet that includes option -82 information to an untrusted port.