Dell POWEREDGE M1000E - Manual (Page 325)

DHCP Snooping Commands 325
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DHCP Snooping Commands
DHCP Snooping is a security feature that monitors DHCP messages between
DHCP clients and DHCP server to filter harmful DHCP messages and build
a bindings database of {MAC address, IP address, VLAN ID, interface} tuples
that are considered authorized.
The DHCP snooping application processes incoming DHCP messages. For
DHCPRELEASE and DHCPDECLINE messages, the application compares
the receive interface and VLAN with the client's interface and VLAN in the
bindings database. If the interfaces do not match, the application logs the
event and drops the message. For valid client messages, DHCP snooping
compares the source MAC address to the DHCP client hardware address.
When there is a mismatch, DHCP snooping logs and drops the packet.
DHCP Snooping forwards valid client messages on trusted members within
the VLAN. If DHCP Relay and/or DHCP Server coexist with DHCP
Snooping, the DHCP client message is sent to the DHCP Relay or/and
DHCP Server for further processing.
The DHCP Snooping application uses DHCP messages to build and
maintain the binding's database. The binding's database only includes data
for clients on untrusted ports. DHCP Snooping creates a tentative binding
from DHCP DISCOVER and REQUEST messages. Tentative bindings tie a
client to a port (the port where the DHCP client message was received).
Tentative bindings are completed when DHCP Snooping learns the client's IP
address from a DHCP ACK message on a trusted port. DHCP Snooping
removes bindings in response to DECLINE, RELEASE, and NACK messages.
The DHCP Snooping application ignores the ACK messages as a reply to the
DHCP Inform messages received on trusted ports. The network administrator
can enter static bindings into the binding database.
IP Source Guard and Dynamic ARP Inspection use the DHCP Snooping
bindings database for the validation of IP and ARP packets.
Commands in this Chapter
This chapter explains the following commands:
2CSPC4.XModular-SWUM200.book Page 325 Thursday, March 10, 2011 11:18 AM