Planet IGSW-2840 User Manual page 481

show ip dhcp snooping
show ip dhcp snoopingbinding
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 4-148), DHCP messages received on an untrusted interface (as specified by the no ip dhcp snooping trust
command, page 4-149) 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 the 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 mac-address
command, page 4-150). However, if MAC address verification is enabled, then the packet will only be forwarded if the
Shows the DHCP snooping configuration settings
Shows the DHCP snooping binding table entries
Table 5-44 DHCP Snooping Commands
481
User's Manual of IGSW-2840
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