Edge-Core ECS4660-28F Management Manual page 444

| Security Measures
C 13
HAPTER
DHCP Snooping
DHCP S
NOOPING
The addresses assigned to DHCP clients on insecure 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.
C U
OMMAND 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.
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