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DHCP Snooping
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.
Command Usage
DHCP Snooping Process
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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.
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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.
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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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Chapter 12
| Security Measures
DHCP Snooping
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