Installation manual

3-116
SIGNAMAX LLC • www.signamax.eu
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:
* 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.