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271 Cisco Small Business 300 Series Managed Switch Administration Guide
16
STEP 6 Device forwards DHCPOFFER, DHCPACK, or DHCPNAK.
The following summarizes how DHCP packets are handled from both trusted and
untrusted ports. The DHCP Snooping Binding database is stored in non-volatile
memory.
DHCP Snooping Packet Handling
Packet Type Arriving from
Untrusted Ingress
Interface
Arriving from Trusted Ingress
Interface
DHCPDISCOVER Forward to trusted
interfaces only.
Forwarded to trusted interfaces
only.
DHCPOFFER Filter. Forward the packet according to
DHCP information. If the
destination address is unknown
the packet is filtered.
DHCPREQUEST Forward to trusted
interfaces only.
Forward to trusted interfaces
only.
DHCPACK Filter. Same as DHCPOFFER and an
entry is added to the DHCP
Snooping Binding database.
DHCPNAK Filter. Same as DHCPOFFER. Remove
entry if exists.
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