Users Guide

Network Administration: DHCP Snooping and DHCP Relay 303
FILE LOCATION: C:\Users\gina\Desktop\Checkout_new\Dell Astute\User
Guide\Dell_Astute_Network_Admin_DHCP_Snooping&Relay.fm
DELL CONFIDENTIAL – PRELIMINARY 8/9/16 - FOR PROOF ONLY
As shown in Table 18-1, the DHCP Snooping Binding database is updated by
interception of DHCPACK, DHCPDECLINE and DHCPRELEASE packets,
and is stored in non-volatile memory.
Even if a port is down, its entries are not deleted.
NOTE: Only DHCP requests on untrusted ports are maintained in the Binding
database .
Limitations
The following limitations apply:
• Enabling DHCP snooping uses TCAM resources.
• The switch writes changes to the binding database only when the switch
system clock is synchronized with SNTP.
• The switch does not update the Binding database when a station moves to
another interface.
DHCPDECLINE Check if there is
information in the
database. If the
information exists and
does not match the
interface on which the
message was received, the
packet is filtered.
Otherwise the packet is
forwarded to trusted
interfaces only, and the
entry is removed from
database.
Forward to trusted
interfaces only
DHCPRELEASE Same as
DHCPDECLINE.
Same as
DHCPDECLINE.
DHCPINFORM Forward to trusted
interfaces only.
Forward to trusted
interfaces only.
DHCPLEASEQUERY Filtered. Forward.
Table 18-1. DHCP Packet Handling when DHCP Snooping is Enabled
(continued)
Packet Type Arriving from Untrusted
Ingress Interface
Arriving from Trusted
Ingress Interface