Samsung iES4028F Management Manual page 502

4
Command Line Interface
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 4-168). However, if
MAC address verification is enabled, then the packet will only be
forwarded if the client's hardware address stored in the DHCP packet is
the same as the source MAC address in the Ethernet header.
* If the DHCP packet is not a recognizable type, it is dropped.
- If a DHCP packet from a client passes the filtering criteria above, it will only
be forwarded to trusted ports in the same VLAN.
- If a DHCP packet from a server is received on a trusted port, it will be
forwarded to both trusted and untrusted ports in the same VLAN.
• If the DHCP snooping is globally disabled, all dynamic bindings are removed
from the binding table.
• Additional considerations when the switch itself is a DHCP client – The port(s)
through which the switch submits a client request to the DHCP server must be
configured as trusted (ip dhcp snooping trust, page 4-167). Note that the
switch will not add a dynamic entry for itself to the binding table when it
receives an ACK message from a DHCP server. Also, when the switch sends
out DHCP client packets for itself, no filtering takes place. However, when the
switch receives any messages from a DHCP server, any packets received
from untrusted ports are dropped.
Example
This example enables DHCP snooping globally for the switch.
Console(config)#ip dhcp snooping
Console(config)#
Related Commands
ip dhcp snooping vlan (4-167)
ip dhcp snooping trust (4-167)
4-166