Korenix JetNet 5228G Series User Manual page 156

3
Configuring the Switch
• 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.
- If a DHCP packet is from server is received on a trusted port, it will be forwarded
to both trusted and untrusted ports in the same VLAN.
• If the DHCP snooping is globally disabled, all dynamic bindings are removed from
the binding table.
• Additional considerations when the switch itself is a DHCP client – The port(s)
through which the switch submits a client request to the DHCP server must be
configured as trusted. Note that the switch will not add a dynamic entry for itself to
the binding table when it receives an ACK message from a DHCP server. Also,
when the switch sends out DHCP client packets for itself, no filtering takes place.
However, when the switch receives any messages from a DHCP server, any
packets received from untrusted ports are dropped.
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