Installation manual

4-163
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ip dhcp snooping
This command enables DHCP snooping globally. Use the no form to restore the default
setting.
Syntax
[no] ip dhcp snooping
Default Setting
Disabled
Command Mode
Global Configuration
Command Usage
• Network traffic may be disrupted when malicious DHCP messages are received
from an outside source. DHCP snooping is used to filter DHCP messages received
on an unsecure interface from outside the network or firewall. When DHCP
snooping is enabled globally by this command, and enabled on a VLAN interface
by the ip dhcp snooping vlan command (page 4-164), DHCP messages received
on an untrusted interface (as specified by the no ip dhcp snooping trust
command, page 4-165) from a device not listed in the DHCP snooping table will be
dropped.
• When enabled, DHCP messages entering an untrusted interface are filtered based
upon dynamic entries learned via DHCP snooping.
• Table entries are only learned for untrusted interfaces. Each entry includes a MAC
address, IP address, lease time, VLAN identifier, and port identifier.
• When DHCP snooping is enabled, the rate limit for the number of DHCP messages
that can be processed by the switch is 100 packets per second. Any DHCP packets
in excess of this limit are dropped.
• Filtering rules are implemented as follows:
- If global DHCP snooping is disabled, all DHCP packets are forwarded.
- If DHCP snooping is enabled globally, and also enabled on the VLAN where the
DHCP packet is received, all DHCP packets are forwarded for a trusted port. If
the received packet is a DHCP ACK message, a dynamic DHCP snooping entry
is also added to the binding table.
- If DHCP snooping is enabled globally, and also enabled on the VLAN where the
DHCP packet is received, but the port is not trusted, it is processed as follows:
* If the DHCP packet is a reply packet from a DHCP server (including OFFER,
ACK or NAK messages), the packet is dropped.
* If the DHCP packet is from a client, such as a DECLINE or RELEASE
message, the switch forwards the packet only if the corresponding entry is
found in the binding table.
* If the DHCP packet is from client, such as a DISCOVER, REQUEST,
INFORM, DECLINE or RELEASE message, the packet is forwarded if MAC
address verification is disabled (as specified by the ip dhcp snooping verify