PLANET SGSD-1022 - Manual (Page 325)

User’s Manual of SGSD-1022 / SGSD-1022P
SGSW-2840 / SGSW-2840P
4.11.15 DHCP Snooping
The addresses assigned to DHCP clients on unsecure ports can be carefully controlled using the dynamic bindings registered
with DHCP Snooping (or using the static bindings configured with IP Source Guard). DHCP snooping allows a switch to protect
a network from rogue DHCP servers or other devices which send port-related information to a DHCP server. This information
can be useful in tracking an IP address back to a physical port.
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 a non-secure interface from outside the network or firewall. When DHCP snooping
is enabled globally and enabled on a VLAN interface, DHCP messages received on an untrusted interface from a device not
listed in the DHCP snooping table will be dropped.
• 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.
• 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.
• 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:
o
If the DHCP packet is a reply packet from a DHCP server (including OFFER, ACK or NAK messages), the packet
is dropped.
o
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.
o
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.
o
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.
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