User's Manual

User’s Manual of GS-4210-24P2S
207
 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.
4.9.3.2 Global Setting
DHCP Snooping is used to block intruder on the untrusted ports of switch when it tries to intervene by injecting a bogus DHCP
reply packet to a legitimate conversation between the DHCP client and server. Configure DHCP Snooping on this page. The
DHCP Snooping Setting and Information screens in Figure 4-9-14 & Figure 4-9-15 appear.
Figure 4-9-14: DHCP Snooping Setting Page Screenshot