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Chapter 4: Web configuration
NS3562-8P-2S User Manual 157
Command usage
Table entries are only learned for trusted interfaces. An entry is added or removed
dynamically to the DHCP snooping table when a client receives or releases an IP
address from a DHCP server. Each entry includes a MAC address, IP address, lease
time, VLAN identifier, and port identifier.
When DHCP snooping is enabled, DHCP messages entering an untrusted interface are
filtered based upon dynamic entries learned via DHCP snooping.
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.