Interlogix NS3500-28T-4S, User Manual

Page: 253 / 363

253

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.
 If the DHCP packet is from a client, such as a DISCOVER, REQUEST, INFORM, DECLINE or
RELEASE message, the packet is forwarded if MAC address verification is disabled. 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 is from 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. 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.