LevelOne GTL-2691 - Manual (Page 397)

C HAPTER 13 | Security Measures
DHCP Snooping
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C OMMAND U SAGE
DHCP Snooping Process
◆ 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 fire wall. 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:
■
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.